feat(agent): tiered validation pipeline (Tiers 2–3)

**Context:** ROADMAP.md → Agent quality → Tiered validation pipeline
**Related:** #149 (ADR-013 closed), in-pipeline fix-up draft, evaluation pipeline draft

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## Component

Agent (Python runtime)

## Describe the feature

Three post-agent tiers per roadmap:

1. **Tier 1 (already shipped):** tool validation—build/test/lint via agent post-hooks and PR body baselines. *This issue implements Tiers 2–3 only.*
2. **Tier 2:** code quality—DRY/SOLID/complexity analysis.
3. **Tier 3:** risk and blast radius analysis—feeds PR risk classifier and review routing (#425).

See ADR-013 tiered validation pyramid.

## Use case

Build/lint passing does not imply maintainable or low-risk changes. Operators want graduated validation before PR creation and to feed evaluation metrics.

## Proposed solution

1. Define tier interfaces in `agent/` with Blueprint opt-in per repo.
2. Tier 2: integrate existing linters/analyzers (eslint complexity, ruff, optional semgrep rules) with structured JSON output.
3. Tier 3: rule-based blast radius (files touched, public API surface, migrations)—output consumed by PR risk classifier draft.
4. Non-blocking advisory mode first; optional Cedar/policy gates later.
5. Emit validation results to `TaskEventsTable` and task record for dashboards.

## Other information

- ADR-013 documents the validation pyramid; this issue tracks **implementation**.
- Design context: `docs/decisions/ADR-013-tiered-validation-pyramid.md`, `docs/design/EVALUATION.md`.

- [ ] This might be a breaking change

Start by reading docs/decisions/ADR-013-tiered-validation-pyramid.md and docs/design/EVALUATION.md, then inspect the agent/ interfaces and the TaskEventsTable references. Define how Tiers 2–3 integrate with Blueprint opt-in, structured advisory results, blast-radius output, and task records; done means these results can feed dashboards and the PR risk classifier.

Written by the indexing model from the issue text.