docs: streamline agent instructions - #28243

Summary

Streamlines the agent instruction files, cutting per-session token overhead by roughly 65%.

The core design change: AGENTS.md no longer @-imports WORKFLOWS.md, package.json, the eight detailed guides, or AGENTS.local.md into every session. Guidance moves from guaranteed-in-context to load-on-demand through a routing table, betting that one-line guardrail summaries plus links are enough to make agents pull the right doc when a task needs it.

• AGENTS.md (251 -> 80 lines): a routing table to the canonical docs in .claude/docs/, the essential command table, and one-line guardrails for the architectural footguns (SQLC workflow plus audit table, OAuth2 error conformance, chatd architecture-doc rule, Swagger annotations, InTx discipline, test concurrency, workflow permissions: {}, mandatory git hooks, commit/PR title format).
• site/AGENTS.md (364 -> 89 lines): keeps the FE1-FE10 contract summary, TypeScript LSP navigation, pnpm command table, React Compiler boundaries, accessibility rules, and the pre-PR frontend-review requirement. Drops the generic React tutorial sections that restate framework documentation.
• The persona and relationship prose ("Our relationship", foundational rules) is deliberately removed. The behavioral gates worth keeping for every agent survive as one-liners: get explicit permission before excepting a rule, discuss architectural decisions before implementing, and answer questions instead of jumping to implementation.
• Unique operational content is relocated rather than deleted: Playwright failure-artifact paths and the CI playwright-artifacts- naming convention move to .claude/docs/AGENT_FAILURES.md; the React.useId() rule, the key={String(booleanState)} remount footgun, date-as-prop Storybook determinism, and the suppression-comment rule (updated from eslint-disable to biome-ignore to match current tooling) move to .claude/docs/FRONTEND_PATTERNS.md.

Content reflects current main: the chatd rule says agents leave TODOs and the human PR author writes the architecture doc, and MUI/Emotion are described as removed, not deprecated.

Note for Claude Code users: CLAUDE.md symlinks to AGENTS.md, so detailed workflow docs (hook internals, the new-feature checklist, AGENTS.local.md) are no longer auto-loaded into every session; agents must follow the links.

Validation

• pnpm exec markdownlint-cli2 on all four touched files: 0 errors
• pnpm exec markdown-table-formatter --check on all four touched files: pass
• make lint/emdash: pass
• make lint/agents: pass
• All relative links verified to resolve

Mux created this PR and description on behalf of Mike.