docs: update index.md for in-memory store removal [doc-updater]

[james-in-a-box]

Update docs/index.md summary entry for coordination-state.md to reflect that the in-memory message store backend was removed.

The commit abe02f8 ("Remove the in-memory message store backend #3159") already updated docs/architecture/coordination-state.md, docs/guides/concurrent-execution.md, and docs/development/STRUCTURE.md. However, docs/index.md still described the coordination-state doc as covering "accidental mid-phase memory-backend loss" as an active concern. The phrase "vs accidental mid-phase memory-backend loss" implied an ongoing risk; the updated text notes it is retired with the #3159 removal.

Triggered by: abe02f8 (Remove the in-memory message store backend #3159 / #3170)

Authored-by: egg

[egg-reviewer]

Approve. Accurate, well-scoped documentation sync. Verified thoroughly:

• Claim is correct. The new summary states that "accidental mid-phase memory-backend loss" was "retired with [#3159] Redis-only removal." This is fully consistent with the target doc docs/architecture/coordination-state.md:173, which classifies the in-memory MessageStore backend as a Retired surface ("#3159 removed the in-memory backend; Redis Streams is the only backend and creation fails loudly rather than degrading"). Lines 182–187 of that doc corroborate the Redis-only end state.
• Replacement reads correctly. The old phrase vs accidental mid-phase memory-backend loss implied an ongoing risk; the new parenthetical correctly marks it as retired. The _clear_concurrent_state() designed phase-boundary wipe is still described as the active, required behaviour — matching coordination-state.md:172.
• Markdown is valid. The edited row keeps the correct 3-pipe structure for the 2-column table; no cell boundaries broken.
• Link format matches convention. [#3159](https://github.com/jwbron/egg/issues/3159) follows the same [#NNNN](…/issues/NNNN) pattern used elsewhere in index.md.

No correctness, security, or rendering issues. The PR body's rationale matches the actual diff.

— Authored by egg

[james-in-a-box]

egg review completed. View run logs