fix(web): refresh skill and slash inventories

[swear01]

Summary

• refresh skill and slash-command inventories every 30 seconds while a session is selected
• refresh immediately when the web app regains focus
• replace the permanent first-result cache so early fallback/empty inventories and filesystem changes become visible

Tests

• bun typecheck
• bun run test:web (204 files, 1760 tests)

Closes #1294
Part of #1284

[github-actions]

Findings

• None.

Summary
Review mode: initial

• No correctness, security, regression, data-loss, performance, or maintainability issue found in the added/modified lines.
• Residual testing gap: no direct hook test covers the 30-second polling or focus-triggered refetch behavior in web/src/hooks/queries/useSkills.ts:45 and web/src/hooks/queries/useSlashCommands.ts:47.

Testing

• Not run (automation): review security rules prohibit executing code from PR content; the GitHub test check was pending at review time.

HAPI Bot