docs(pr-review): plan specs 12–16 and ADRs 0016–0018

[orioltf]

Summary

• Lands planning docs only — no source changes, no behaviour change. Five new specs (12–16) and three new ADRs (0016–0018) for the pr-review plugin, plus the index/cross-reference touches that make them discoverable.
• Spec 12 (P0): fold thread fetch + mode detection into the ADO Fetcher, fixing the broken az repos pr thread list Step 4 that has been silently breaking ADO PR reviews. Spec 14: ADO CLI preflight assertion + smoke test with an inventory-completeness check. Specs 13, 15, 16 (dry-run mode, fan-out resilience Notice, aspect-selection observability) are written and indexed but deferred to a follow-up implementation PR.
• ADR 0016 absorbs two amendments in one place: ADR 0013's "one allowed inline ADO call" carve-out removal, and ADR 0015's thread-fetch ABORT exemption (justified by CONTEXT.md's mode-misdetection rule). CONTEXT.md's ADO Fetcher definition widened to reflect the new responsibility.
• PRD published as PRD: Restore ADO PR review path + ADO CLI smoke test (specs 12 + 14) #120; this branch is the planning commit referenced from there. Implementation for specs 12 + 14 will land in a separate PR.

Test plan

• CI green on this branch (docs-only changes; nothing to execute).
• Reviewer reads through ADR 0016 and confirms the joint amendment scope (0013 carve-out removal + 0015 thread-fetch ABORT exemption) is clearer in one document than two would be.
• Reviewer cross-checks docs/plans/README.md and docs/adr/README.md index rows against the actual files (12–16 / 0016–0018) and confirms no broken links.
• Reviewer reads the updated ADO Fetcher entry in CONTEXT.md and confirms the new wording covers thread fetch + mode detection without contradicting any other Relationship line.

Refs #120

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