fix: scan staged lesson files in reverse lint (v1.26.0)

[wan-huiyan]

Summary

• include git diff --cached --name-only in reverse-lint file discovery
• add an end-to-end regression that stages lessons.md without committing and proves exactly one file is scanned
• release the fix as v1.26.0 across all manifests and README history

Test plan

• node --test tests/dependency-path-resolution.test.mjs (47/47)
• npm test (80 total; 78 pass, 2 intentional eval-suite skips)
• pre-push npm test gate (same result)
• git diff --check

Diff

7 files changed, 33 insertions, 9 deletions.

[wan-huiyan]

Independent review of immutable commit 204fd16 completed on two axes.

• Spec: 0 findings. The staged-file collector, exact one-file regression, isolated-worktree workflow, release metadata, focused tests, and cache boundary all match the request.
• Standards: 0 violations. One non-blocking duplication nit in the test fixture setup was considered and left unchanged because the setup is short and keeps the staged-only regression self-contained.

All local and GitHub checks are green.