Add .cursorrules link and include more in REVIEW.md about commit series (#176)

* common: Add .cursorrules symlink pointing to AGENTS.md

Cursor reads .cursorrules as its agent instruction file. Symlinking it
to AGENTS.md keeps a single source of truth rather than maintaining
separate files that can drift.

Assisted-by: OpenCode (claude-sonnet-4-6@default)
Signed-off-by: Colin Walters <walters@verbum.org>

* REVIEW: Clarify that review feedback should be squashed, not a new commit

A recurring pattern, especially with AI-assisted reviews (Gemini, Copilot,
etc.), is that applying a suggested fix creates a new commit with a generic
auto-generated subject like "Update README.md". That commit should be
squashed into the original before merge.

Motivated by bootc-dev/bootc#2182.

Assisted-by: OpenCode (claude-sonnet-4-6@default)
Signed-off-by: Colin Walters <walters@verbum.org>

---------

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

common/.cursorrules

@@ -0,0 +1 @@
+AGENTS.md

common/REVIEW.md

@@ -139,6 +139,20 @@ Generally, just restate the commit message.
 
 Where it makes sense, it is OK to include additional details though.
 
+### Further changes on top of existing commits
+
+If you have followup fixes (whether that's part of a local loop or
+as part of addressing PR review), it is generally encouraged to *squash*
+the fixes into the prior commit. Do not create generically-named "Update <file>" commits
+or "Address review feedback" or "Fix cargo fmt" commits.
+
+This applies equally when an AI tool (e.g. Gemini, Copilot) suggests a
+change via a review comment — applying the suggestion creates a new commit
+with an auto-generated subject. That commit should be squashed before the
+PR is merged.
+
+In other words either a commit "stands alone" with its own rationale or it doesn't.
+
 ### Keeping PRs Current
 
 Keep PRs rebased on main. When CI failures are fixed in other PRs, rebase to