fix(mcp): resolve active session from store for mem_save instead of manual-save fallback

[Alan-TheGentleman]

Summary

Closes #386 — mem_save (and prompt/summary/passive saves) without an explicit session_id fell back to manual-save-{project} even when a real UUID session was active, leaving UUID sessions at observation_count: 0.

Root cause

engram serve (HTTP) and engram mcp (stdio) are separate processes sharing only the on-disk SQLite store. The SessionStart hook registers the UUID session via POST /sessions (HTTP process), so any in-process state in the MCP process stays empty in the real flow.

Change

• New store method MostRecentActiveSession(project) — resolves the most-recent un-ended session from the shared sessions table (ended_at IS NULL, excludes manual-save%, ORDER BY started_at DESC).
• New resolveFallbackSessionID helper wired into the 4 MCP fallback sites; falls back to manual-save-{project} only when no active session exists or the query errors (fail-safe).

Test plan

TDD red→green. Covers the real repro (session created as POST /sessions does → mem_save with no session_id lands in the UUID session), plus no-active-session fallback, multi-session recency, ended-session skip, manual-save% exclusion, project scoping. go test -race ./internal/mcp/... clean; go build ./... clean.

Notes

Passed adversarial review (cross-process correctness, fail-safe, determinism verified). Non-blocking follow-up: a sessions(project, ended_at) index if session volume grows.

[Copilot]

Pull request overview

This PR fixes MCP write tools (mem_save, prompt/summary/passive capture) defaulting to manual-save-{project} even when a real UUID session is active in the shared SQLite store, by resolving the most-recent active session directly from persisted sessions rows.

Changes:

• Add Store.MostRecentActiveSession(project) to select the latest un-ended, non-manual-save% session for a project.
• Wire a new resolveFallbackSessionID helper into MCP write-tool fallback paths so implicit writes prefer the persisted active UUID session.
• Add store- and MCP-level tests covering active-session resolution, ended-session exclusion, recency ordering, and preserved manual-save fallback.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
internal/store/store.go	Adds MostRecentActiveSession query for cross-process active session resolution.
internal/store/store_test.go	Adds unit tests validating active-session selection rules and project scoping.
internal/mcp/mcp.go	Uses store-backed fallback session resolution for write tools when session_id is omitted.
internal/mcp/mcp_test.go	Adds regression tests reproducing issue #386 and verifying new fallback behavior.

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