feat(schemas): dynamic schema enrichment lifecycle (#274)

[tikazyq]

Closes #274

Summary

Formalises the resolve_schema() lifecycle so long-running processes don't pay the schema-enrichment network round-trip on every request, and so offline / bad-token sessions degrade gracefully instead of aborting.

• core: new AdapterCache (TTL-keyed by project root) — get_or_resolve, refresh, invalidate, clear. resolve_inline failures inside AdapterRegistry::create are now warnings on stderr; the CLI falls back to the static schema instead of refusing to run.
• http: AppState carries the shared Arc<AdapterCache>. Spec / adapter / schema handlers route through resolve_adapter_cached. New POST /api/projects/{id}/schema/refresh flushes a project's cache entry and returns the freshly-resolved schema.
• mcp: ServerState holds the adapter behind a RwLock<Arc<dyn Adapter>> so the new reload_schema tool can swap in a freshly-resolved adapter without restarting the server.
• cli: leanspec capabilities --refresh (informational on the single-shot CLI; surfaces enrichment errors loudly when present).

Provider impact

Touches the adapter seam. The Adapter trait surface is unchanged; what changed is how callers hold the resolved adapter:

Before	After
Box<dyn Adapter> returned by resolve_adapter everywhere	HTTP handlers receive Arc<dyn Adapter> from resolve_adapter_cached (shared, re-usable); MCP keeps ServerState::adapter() returning a snapshot Arc<dyn Adapter>; CLI still gets Box<dyn Adapter> (single-shot, no cache benefit).
resolve_inline errors in AdapterRegistry::create were fatal	Errors emit warning: could not enrich <adapter> schema (…). Using static defaults. on stderr and the adapter falls back to its declared defaults.

No serialized format / public-API breakage; UI / agents that already call GET /api/projects/{id}/schema keep working unchanged.

Test plan

• cargo test --all-features (all suites green, including new AdapterCache unit tests, HTTP schema/refresh integration tests, and MCP reload_schema dispatch test)
• cargo clippy --all-features --all-targets -- -D warnings
• cargo fmt --check
• Cache: TTL expiry, invalidate, refresh, clear, multi-project isolation
• HTTP: schema refresh returns fresh payload; unknown-project 404; back-to-back GETs return identical schemas (cache hit)
• MCP: reload_schema tool advertised in tools/list and dispatchable
• Manual: leanspec capabilities --refresh on a GitHub-configured project (left for follow-up — requires a real GitHub token; covered by the existing create_github_adapter_resolves_labels mockito test).

Delivers

From the Plan in #274:

• Define resolve_schema() lifecycle in AdapterRegistry::from_project() (treat errors as warnings)
• Add AdapterRegistry instance cache for long-running processes (AdapterCache with TTL, default 5 minutes)
• HTTP server: cached adapter resolution
• HTTP server: POST /api/projects/{id}/schema/refresh
• MCP server: reload_schema tool
• CLI: --refresh flag on capabilities
• Document enrichment behavior (CHANGELOG entry)

Note: the spec's example output of "(14 options)" after enum field lines is not yet rendered by the capabilities command — the resolved options are already shown in the existing enum [open, closed] form, so the missing piece is purely cosmetic and was deferred to keep this PR focused on the lifecycle / caching mechanics.

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