Correct stale README and guide claims about resume storage and API names

[nficano]

Several prose docs describe behavior or files that do not match the current implementation. README.md:30 and arcp.gemspec:14 say the SDK ships a SQLite-backed resume log, while lib/arcp/runtime/event_log.rb:5 through lib/arcp/runtime/event_log.rb:9 says v1 ships an in-memory event log and no SQLite-backed log implementation exists. README.md:98 through README.md:126, docs/guides/resume.md, and docs/recipes.md:228 through docs/recipes.md:261 present transparent resume replay as current behavior even though the runtime ignores the resume payload. docs/guides/leases.md:46 names BudgetCounter#try_spend!, but the method is Arcp::Runtime::LeaseManager#try_spend! and the lower-level counter method is try_decrement. docs/architecture.md:176 links to guides/agent-versioning.md, which is not present in the repository, and docs/architecture.md:165 through docs/architecture.md:167 says an unavailable agent version populates details['available_versions'] even though the runtime uses the key available.

Fix prompt: Audit README.md, arcp.gemspec, CONFORMANCE.md, docs/guides/*.md, docs/architecture.md, docs/recipes.md, and the generated diagram sources against the current Ruby code and either update the implementation or correct the prose so it no longer overstates shipped features. The docs should distinguish implemented in-memory event buffering from any planned SQLite persistence, remove or mark unsupported resume examples until the runtime actually resumes sessions, use the exact Ruby method names and error detail keys that exist, and replace broken links with existing pages or add the missing guide.