docs: one golden path across PR template, CONTRIBUTING, GEMINI, AGENTS + CI milestones in ROADMAP (#8380)

Contributor guidance contradicted itself in four places (found in the #8084 review):

- pull_request_template.md + CONTRIBUTING.md asked contributors to run the FULL
  unit suite + coverage gate locally, while the maintainer's stated golden path
  (#8273/#8329) is: focused tests for the change locally; full suite, coverage,
  and build are CI's job. On 16GB hosts the full local chain has saturated
  machines (#8084 incident report).
- GEMINI.md demanded coverage >= 75/75/75/70 while the official CI gate is
  60/60/60/60 (quality-baseline ratchet on top).
- AGENTS.md fork workflow said to branch from upstream/main; the default branch
  is the active release/vX.Y.Z line (main only receives release squash-merges).

Also makes the #8084 CI direction explicit in the public ROADMAP: lane
consolidation (3.8.51), one CI policy for release/** and main (3.8.52),
full-regression authority -> merge queue after TIA shadow evidence (3.8.54),
preview-artifact + build-once rehearsal inside the 3.8.58 dry-run.

Refs #8329
Refs #8084

.github/pull_request_template.md

@@ -9,10 +9,12 @@
 
 ## Validation
 
+Run only the focused loop for what you changed — the full unit suite, Vitest, the
+60% coverage gate, and the production build all run in CI on this PR (#8329):
+
+- [ ] Focused tests for the change: `node --import tsx/esm --test tests/unit/<file>.test.ts`
 - [ ] `npm run lint`
-- [ ] `npm run test:unit`
-- [ ] `npm run test:coverage`
-- [ ] Coverage is still `>= 60%` for statements, lines, functions, and branches
+- [ ] Production-code changes include a new or updated automated test in this PR
 - [ ] SonarQube PR analysis is green or any remaining issues are explicitly documented below
 
 ## Tests Added Or Updated