feat(ci): add make pre-pr target and fixture staleness check

Implements make pre-pr chain: lint -> build -> test -> content
validation -> fixture staleness. Branch-specific variants:
pre-pr-server, pre-pr-client, pre-pr-content.

Fixture staleness is a blocker (exit 1) — stale fixtures cause
false positive client tests. Spec from hoshe-round3.md Section 5.

Tickets: #460, #465

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/DEVOPS.md

@@ -89,6 +89,34 @@ make ci-client          # lint-client → build-client → test-client
 
 CI targets chain lint → build → test sequentially. A failure in any stage stops the pipeline.
 
+### Pre-PR Checks
+
+Before pushing a PR, run:
+
+```bash
+make pre-pr
+```
+
+This runs all checks in order: lint → build → test → content validation → fixture staleness. Total runtime ~2.5 minutes (incremental build), under 3 minutes clean.
+
+For branch-specific checks:
+
+```bash
+make pre-pr-server      # Server changes: lint, build, test, fixture staleness
+make pre-pr-client      # Client changes: lint, build, test
+make pre-pr-content     # Content changes: schema + cross-reference validation
+```
+
+If `pre-pr-fixtures` fails, your protocol changes require fixture regeneration:
+
+```bash
+make fixtures
+git add client/tests/fixtures/
+git commit -m "chore(fixtures): regenerate for protocol vN"
+```
+
+The fixture staleness check is a **blocker** (exit 1) — stale fixtures cause false positive client tests.
+
 ### Clean
 
 ```bash