test: add SessionManager unit coverage

[edgepillar]

Summary

• Add Vitest as the test runner and replace the placeholder npm test command with vitest run.
• Add five deterministic unit tests for SessionManager.
• Update the testing documentation in README.md, docs/ARCHITECTURE.md, and CLAUDE.md.

Test coverage

• Retain and return an unlocked opaque keystore.
• Lock one address without locking another.
• Remove all sessions with lockAll().
• Lazily evict an expired session when it is accessed.
• Filter expired sessions while retaining a more recently unlocked session.

Validation

• npm test — passed (1 test file, 5 tests).
• npm run typecheck — passed with zero errors.
• npm run lint — passed with zero errors and 6 existing warnings.
• npm run build — passed.
• npm run build:extension — passed.
• npm audit --audit-level=critical — passed with zero critical advisories.
• git diff --check — passed.

Notes

• Production SessionManager behavior is unchanged.
• No CI workflow is added or changed.
• This is a follow-up to the testing discussion in PR feat: refactor service classes #14.