feat(skills): hoist call-adcp-agent + bundle skills/ into protocol tarball

[bokelley]

Summary

• New skills/call-adcp-agent/SKILL.md — agent-facing buyer-side wire contract (idempotency replay, account oneOf variants, async status:'submitted' polling, adcp_error.issues[] recovery). Hoisted from @adcp/client@5.17.0 and made canonical here so all SDKs (@adcp/client, adcp Python, adcp-go) consume one source of truth.
• Cross-SDK distribution: scripts/build-protocol-tarball.cjs now bundles skills/ into the published /protocol/<version>.tgz. SDKs already pull this tarball at sync time for schemas and compliance — version-pinned, Sigstore-verified, no manual copy. manifest.json.contents.skills is an enumerated list so SDK sync scripts can pick out skill names without re-walking the tree.
• Per-protocol skills (adcp-{brand,creative,governance,media-buy,si,signals}) gain a one-line pointer at the top deferring cross-cutting rules to call-adcp-agent.
• New docs/protocol/calling-an-agent.mdx — human-readable canonical narrative form, registered in docs.json and cited from the SKILL.md (and back).
• Addie wiring: loadSkillDocs is now frontmatter-driven (filters by name: adcp-* or type: cross-cutting) instead of hardcoding directory names. Cross-cutting rules live in a single BUYER_RULES_PREAMBLE injected at the top of every search response. call_adcp_task's tool description trims to the two non-negotiable rules (idempotency_key replay + issues[].variants[] recovery). Failure-path output appends a recovery hint pointing at issues[].
• Two pre-existing prod bugs fixed along the way:
  • Dockerfile was missing COPY /app/skills — skill loader silently returned empty for ALL adcp-* skills in production.
  • Skill loader's hardcoded relative-path traversal landed at /skills (root) in prod (dist/) layout. Replaced with resolveSkillsDir() candidate-list (works in dev, prod, and CWD layouts; exported and unit-tested).
• 12 new tests in tests/addie/buyer-skill-wiring.test.ts lock down skill content, frontmatter version pin, loader resolution, search preamble injection, and tool description shape. Pre-commit hook ran 832 tests + typecheck — all passed.

Live test

Spun up docker compose up, logged in as admin dev user, asked Addie:

"I want to call create_media_buy on test-agent.adcontextprotocol.org. What cross-cutting rules do I need to be aware of as a buyer?"

She called ask_about_adcp_task, cited skills/call-adcp-agent/SKILL.md as the source, and gave all five buyer rules verbatim — including the correct oneOf variants {account_id} OR {brand:{domain}, operator}. Anonymous tier (no AdCP tools exposed) had hallucinated account_name+account_email. The wiring is real.

Pre-PR review

Five experts in parallel — agentic-product-architect, docs-expert, prompt-engineer, dx-expert, code-reviewer. Addressed all "must-fix" + most "should-fix" feedback before commit.

Test plan

• npx vitest run tests/addie/ — 414 passing (was 402 before, +12 from new wiring tests + dropped scope tests)
• npx tsc --project server/tsconfig.json --noEmit — clean
• Pre-commit hook (npm run test:unit && npm run typecheck) — 832 tests passed
• Live Addie conversation in docker compose — buyer rules surface correctly via ask_about_adcp_task
• npm run build:protocol-tarball — verified adcp-latest/skills/call-adcp-agent/SKILL.md is in the tarball; manifest enumerates skills

Follow-ups (not in this PR)

1. SDK-side wire-up to extract skills/ from the tarball at sync time:
   • @adcp/client scripts/sync-schemas.ts
   • adcp-client-python equivalent
   • adcp-go equivalent
2. Once SDKs pull from canonical, remove the local copy from @adcp/client/skills/call-adcp-agent/.
3. Per docs-expert: link from per-task .mdx pages (e.g. docs/media-buy/task-reference/create_media_buy.mdx) to the canonical idempotency_key list rather than redefining inline.

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