feat(tui): show per-story commit diff and add pgup/pgdn scrolling

Diff view now shows the commit for the selected user story instead of
the entire branch diff. Uses git log --grep to find the story's commit
by ID. Also adds pgup/pgdn as alternate key bindings for page scrolling
in log and diff views.

Author: MiniCodeMonkey

internal/git/git.go

@@ -125,6 +125,52 @@ func GetDiffStats(dir string) (string, error) {
 	return strings.TrimSpace(string(output)), nil
 }
 
+// GetDiffForCommit returns the diff for a single commit.
+func GetDiffForCommit(dir, commitHash string) (string, error) {
+	cmd := exec.Command("git", "diff", commitHash+"~1", commitHash)
+	cmd.Dir = dir
+	output, err := cmd.Output()
+	if err != nil {
+		// If commitHash~1 doesn't exist (first commit), diff against empty tree
+		cmd = exec.Command("git", "diff", "4b825dc642cb6eb9a060e54bf899d69f82049674", commitHash)
+		cmd.Dir = dir
+		output, err = cmd.Output()
+		if err != nil {
+			return "", err
+		}
+	}
+	return string(output), nil
+}
+
+// GetDiffStatsForCommit returns the diffstat for a single commit.
+func GetDiffStatsForCommit(dir, commitHash string) (string, error) {
+	cmd := exec.Command("git", "diff", "--stat", commitHash+"~1", commitHash)
+	cmd.Dir = dir
+	output, err := cmd.Output()
+	if err != nil {
+		cmd = exec.Command("git", "diff", "--stat", "4b825dc642cb6eb9a060e54bf899d69f82049674", commitHash)
+		cmd.Dir = dir
+		output, err = cmd.Output()
+		if err != nil {
+			return "", err
+		}
+	}
+	return strings.TrimSpace(string(output)), nil
+}
+
+// FindCommitForStory searches the git log for a commit matching the given story ID.
+// Returns the commit hash if found, empty string otherwise.
+func FindCommitForStory(dir, storyID string) (string, error) {
+	cmd := exec.Command("git", "log", "--grep="+storyID, "--format=%H", "-1")
+	cmd.Dir = dir
+	output, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	hash := strings.TrimSpace(string(output))
+	return hash, nil
+}
+
 // getMergeBase returns the merge base commit between two refs.
 func getMergeBase(dir, ref1, ref2 string) (string, error) {
 	cmd := exec.Command("git", "merge-base", ref1, ref2)

internal/tui/app.go

@@ -521,8 +521,19 @@ func (a App) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		// Diff view
 		case "d":
 			if a.viewMode == ViewDashboard || a.viewMode == ViewLog {
+				// Use the current PRD's worktree directory if available, otherwise base dir
+				diffDir := a.baseDir
+				if instance := a.manager.GetInstance(a.prdName); instance != nil && instance.WorktreeDir != "" {
+					diffDir = instance.WorktreeDir
+				}
+				a.diffViewer.SetBaseDir(diffDir)
 				a.diffViewer.SetSize(a.width-4, a.height-headerHeight-footerHeight-2)
-				a.diffViewer.Load()
+				// Load diff for the selected story's commit
+				if story := a.GetSelectedStory(); story != nil {
+					a.diffViewer.LoadForStory(story.ID)
+				} else {
+					a.diffViewer.Load()
+				}
 				a.viewMode = ViewDiff
 			} else if a.viewMode == ViewDiff {
 				a.viewMode = ViewDashboard
@@ -606,13 +617,13 @@ func (a App) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			}
 
 		// Log/diff scrolling
-		case "ctrl+d":
+		case "ctrl+d", "pgdown":
 			if a.viewMode == ViewLog {
 				a.logViewer.PageDown()
 			} else if a.viewMode == ViewDiff {
 				a.diffViewer.PageDown()
 			}
-		case "ctrl+u":
+		case "ctrl+u", "pgup":
 			if a.viewMode == ViewLog {
 				a.logViewer.PageUp()
 			} else if a.viewMode == ViewDiff {

internal/tui/dashboard.go

@@ -736,11 +736,15 @@ func (a *App) renderDiffHeader() string {
 	// Branding
 	brand := headerStyle.Render("chief")
 
-	// View indicator
+	// View indicator - show story ID if viewing a story-specific diff
+	viewLabel := "[Diff View]"
+	if a.diffViewer.storyID != "" {
+		viewLabel = fmt.Sprintf("[Diff: %s]", a.diffViewer.storyID)
+	}
 	viewIndicator := lipgloss.NewStyle().
 		Foreground(PrimaryColor).
 		Bold(true).
-		Render("[Diff View]")
+		Render(viewLabel)
 
 	// State indicator
 	stateStyle := GetStateStyle(a.state)
@@ -787,10 +791,14 @@ func (a *App) renderDiffHeader() string {
 func (a *App) renderNarrowDiffHeader() string {
 	brand := headerStyle.Render("chief")
 
+	viewLabel := "[Diff]"
+	if a.diffViewer.storyID != "" {
+		viewLabel = fmt.Sprintf("[%s]", a.diffViewer.storyID)
+	}
 	viewIndicator := lipgloss.NewStyle().
 		Foreground(PrimaryColor).
 		Bold(true).
-		Render("[Diff]")
+		Render(viewLabel)
 
 	stateStyle := GetStateStyle(a.state)
 	state := stateStyle.Render(fmt.Sprintf("[%s]", a.state.String()))

internal/tui/diff.go

@@ -15,6 +15,7 @@ type DiffViewer struct {
 	height     int
 	stats      string
 	baseDir    string
+	storyID    string // Story ID whose commit diff is being shown (empty = full branch diff)
 	err        error
 	loaded     bool
 }
@@ -32,12 +33,49 @@ func (d *DiffViewer) SetSize(width, height int) {
 	d.height = height
 }
 
-// Load fetches the latest git diff.
+// SetBaseDir updates the base directory used for loading diffs.
+func (d *DiffViewer) SetBaseDir(dir string) {
+	d.baseDir = dir
+}
+
+// Load fetches the latest git diff for the full branch.
 func (d *DiffViewer) Load() {
+	d.storyID = ""
+	d.loadDiff("", "")
+}
+
+// LoadForStory fetches the git diff for a specific story's commit.
+func (d *DiffViewer) LoadForStory(storyID string) {
+	d.storyID = storyID
+
+	// Find the commit for this story
+	commitHash, err := git.FindCommitForStory(d.baseDir, storyID)
+	if err != nil || commitHash == "" {
+		d.offset = 0
+		d.loaded = true
+		d.err = nil
+		d.lines = nil
+		d.stats = ""
+		return
+	}
+
+	d.loadDiff(storyID, commitHash)
+}
+
+// loadDiff loads a diff, either for a specific commit or the full branch.
+func (d *DiffViewer) loadDiff(storyID, commitHash string) {
 	d.offset = 0
 	d.loaded = true
 
-	diff, err := git.GetDiff(d.baseDir)
+	var diff string
+	var err error
+
+	if commitHash != "" {
+		diff, err = git.GetDiffForCommit(d.baseDir, commitHash)
+	} else {
+		diff, err = git.GetDiff(d.baseDir)
+	}
+
 	if err != nil {
 		d.err = err
 		d.lines = nil
@@ -55,9 +93,16 @@ func (d *DiffViewer) Load() {
 
 	d.lines = strings.Split(diff, "\n")
 
-	stats, err := git.GetDiffStats(d.baseDir)
-	if err == nil {
-		d.stats = stats
+	if commitHash != "" {
+		stats, err := git.GetDiffStatsForCommit(d.baseDir, commitHash)
+		if err == nil {
+			d.stats = stats
+		}
+	} else {
+		stats, err := git.GetDiffStats(d.baseDir)
+		if err == nil {
+			d.stats = stats
+		}
 	}
 }
 
@@ -121,6 +166,9 @@ func (d *DiffViewer) Render() string {
 	}
 
 	if len(d.lines) == 0 {
+		if d.storyID != "" {
+			return lipgloss.NewStyle().Foreground(MutedColor).Render("No commit found for " + d.storyID)
+		}
 		return lipgloss.NewStyle().Foreground(MutedColor).Render("No changes detected")
 	}
 

internal/tui/help.go

@@ -90,8 +90,8 @@ func (h *HelpOverlay) GetCategories() []ShortcutCategory {
 			Shortcuts: []Shortcut{
 				{Key: "j / ↓", Description: "Scroll down"},
 				{Key: "k / ↑", Description: "Scroll up"},
-				{Key: "Ctrl+D", Description: "Page down"},
-				{Key: "Ctrl+U", Description: "Page up"},
+				{Key: "Ctrl+D / PgDn", Description: "Page down"},
+				{Key: "Ctrl+U / PgUp", Description: "Page up"},
 				{Key: "g", Description: "Go to top"},
 				{Key: "G", Description: "Go to bottom"},
 			},