chore: bump devx from v0.51.0 to v0.51.9

Picks up DEVX-165 (deps: commit type) and DEVX-166 (docs/plans/* exclusion).

Implements: GRM-168

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/specs/GRM-168.md

@@ -0,0 +1,21 @@
+# GRM-168: Bump devx to v0.51.9
+
+## Problem
+grm pins devx@v0.51.0 which rejects `deps:` as a conventional commit type,
+causing post-merge CI failures on dependency bump commits.
+
+## Approach
+REQ-1: Bump devx from v0.51.0 to v0.51.9 in pyproject.toml
+
+## Test Plan
+- `make lint-all` passes
+- `make pytest-cov` passes
+
+## Deploy Plan
+- Merge to master
+
+## Rollback Plan
+- Revert the merge commit
+
+## Acceptance Criteria
+- [x] REQ-1: Bump devx from v0.51.0 to v0.51.9 in pyproject.toml