feat(index): auto-ship + protect the mandatory escurel meta-skill

[jrosskopf]

Summary

Closes audit finding #1 (spec→impl): the mandatory escurel
meta-skill — the agent's in-corpus documentation of the tool surface,
discovery policy and navigation model (locked decision 3,
docs/contract/agent-interface.md) — was never seeded into tenants and
was unprotected from removal.

• Embed the canonical meta-skill markdown (meta_skill.md) and the
  protection rules (meta_skill.rs).
• Auto-ship via Indexer::ensure_meta_skill() — idempotent, wired
  at indexer open (binary boot through config.rs and the test
  harness) so every served tenant exposes it. No-op when the tenant
  already carries an escurel skill, so operators may ship their own
  extended version (e.g. the crm-demo's CRM-specific one).
• Protect on write: update_page on the meta-skill page rejects a
  rewrite that drops the skill identity (id: escurel, type: skill)
  or removes a section the established meta-skill already carries —
  operators append, never remove. MCP and gRPC return the contract
  {ok:false, issues:[{code:"meta_skill_protected"}]} shape rather than
  a transport error.

Design note: the guard's protected baseline is whatever sections the
established meta-skill has (empty on first write), not a hardcoded
canonical list — so a tenant that legitimately ships a customized
meta-skill (the crm-demo) keeps its own sections protected. create
on the admin TenantStore stays "empty at create time"; the meta-skill
is guaranteed at first serve, which keeps tenant export/import
deterministic.

Test plan

• crates/escurel-index/src/meta_skill.rs unit tests — section
  retention, identity, first-write establishment, append accepted.
• crates/escurel-server/tests/meta_skill.rs (new, no-mock) — a fresh
  tenant ships the meta-skill and its body is expandable; removing a
  standard section is rejected with a meta_skill_protected issue;
  appending tenant guidance is accepted.
• Updated existing skill-count / page-count assertions across the suite
  for the +1 (grpc_read_tools, mcp, client_roundtrip,
  grpc_admin_streaming rebuild, facade).
• Local gate green: cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, cargo test --workspace --all-targets,
  cargo build --workspace --release.

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