docs: move schema-lint-v1-plan into docs/dev/ + add to index

Post-rebase fixup for the docs split (#93). The plan doc was added
to docs/ at the top level before main reorganized to docs/{user,dev}/.
This moves it into docs/dev/ and adds an entry to docs/dev/index.md
under a new "Active Implementation Plans" section so the
check-agents-md.sh link check passes.

Per the original commit message (617a77d), the plan doc is intentionally
temporary — it will be deleted when v1 lands.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/dev/index.md

@@ -51,6 +51,14 @@ constraints. User-facing behavior should still be documented through
 | Install and deployment packaging | [install.md](../user/install.md), [deployment.md](../user/deployment.md) |
 | Release history | [releases/](../releases/) |
 
+## Active Implementation Plans
+
+Working documents for in-flight feature work. Removed when the work lands.
+
+| Area | Read |
+|---|---|
+| Schema-lint chassis v1 (MR-694) — `--allow-data-loss`, soft/hard drops | [schema-lint-v1-plan.md](schema-lint-v1-plan.md) |
+
 ## Boundary
 
 Developer docs may mention implementation details, stale gaps, upstream Lance