docs(plans): add plan docs for OPER-176, OPER-177, POR-778

feature_id: FEAT-studio-agent-rules

## Context

The QA loop tickets OPER-176, OPER-177, and POR-778 are Safeguard tier per `rules/build-risk-matrix.md` and require plan docs at `docs/plans/<TICKET-ID>-<slug>.md` before code execution per `rules/explore-plan-execute.md`.

The three plan docs are drafted in the sandbox at `/home/user/workspace/plans/`:
- `OPER-176-task-risk-tier.md`
- `OPER-177-ai-triage-writer.md`
- `POR-778-playwright-trace-recorder.md`

This ticket ships them into the `operant-studio` repo so the dispatched agents on OPER-176/OPER-177 can reference them. POR-778's plan doc goes into `operant-studio` too (not `portico`) because it references OPER-176/OPER-177 primitives and needs to be co-located with them for cross-repo traceability. When the Portico agent picks up POR-778, it reads the plan from operant-studio via GitHub API.

## Acceptance Criteria

- [ ] New file `docs/plans/OPER-176-task-risk-tier.md` with content from `/home/user/workspace/plans/OPER-176-task-risk-tier.md`
- [ ] New file `docs/plans/OPER-177-ai-triage-writer.md` with content from `/home/user/workspace/plans/OPER-177-ai-triage-writer.md`
- [ ] New file `docs/plans/POR-778-playwright-trace-recorder.md` with content from `/home/user/workspace/plans/POR-778-playwright-trace-recorder.md`
- [ ] Open PR titled `docs(plans): add plan docs for OPER-176, OPER-177, POR-778`
- [ ] Do NOT auto-merge — user reviews and merges

## Risk classification

Sandbox (Risk 2 / Reach 3). Docs-only. No runtime effect. Failure mode: plan drift between what's written and what the dispatched agent builds — caught by human PR review.

## Depends on

None. Ships independently. OPER-176/177 can be admitted before or after this ticket merges (their body will reference the plan doc paths, which resolve once this PR merges).

## Blocks

None strictly, but recommended: OPER-176/OPER-177/POR-778 dispatched agents should reference these plan docs during execution.

## Out of scope

- Editing the plan doc contents (revisions happen in later PRs against the merged files)
- Writing plan docs for OPER-178 (Sandbox tier, no plan doc required per `rules/explore-plan-execute.md`) or OPER-175 (already has a plan at `packages/qa/OPER-125-rerun-plan.md`)


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<!-- admission-rejected -->
**Admission rejected (unknown_feature_id):** feature_id=FEAT-studio-agent-rules is not declared in the feature-map.md of any admission-enabled product owned by this team.

Valid feature_ids for product studio (products/studio/feature-map.md):

- FEAT-studio-agent-workers
- FEAT-studio-audit
- FEAT-studio-audit-log
- FEAT-studio-branch-protection-rulesets
- FEAT-studio-coverage-policy
- FEAT-studio-dependency-map-policy
- FEAT-studio-dispatcher
- FEAT-studio-env-protection
- FEAT-studio-fabricated-evidence-tripwire
- FEAT-studio-guard-inventory
- FEAT-studio-incident-runbook
- FEAT-studio-mutation-testing
- FEAT-studio-phi-guard
- FEAT-studio-pipeline-observability
- FEAT-studio-planning-validation
- FEAT-studio-platform
- FEAT-studio-product-manifest
- FEAT-studio-qa
- FEAT-studio-qa-projects
- FEAT-studio-review-rounds

... and 2 more; see products/studio/feature-map.md