feat: route assistant email reading to native Gmail instead of Composio

[hrsvrn]

Expose the native Gmail stack (sync engine + cached per-thread LLM summaries) to the Copilot as builtin tools, and reroute prompts/skills so email reading, checking, searching, and summarizing no longer go through Composio:

• New gmail-* builtin tools (gmail-checkConnection, gmail-listThreads, gmail-readThread, gmail-searchEmails) backed by sync_gmail snapshots, with LLM-friendly trimmed payloads and a connect-in-Settings payload (+ composioFallback flag) when native Google isn't connected.
• sync_gmail: export getThreadSnapshot, searchThreadsLive, and fetchThreadLive for uncached threads surfaced by live search.
• New read-emails skill: compose inbox overviews from cached summary/importance instead of re-reading bodies; Gmail q-operator search guidance; sync:true for freshness.
• Instructions: nativeGmailConnected flag splits Gmail out of the Composio routing blocks; gmail tools line added; read-emails skill excluded from the catalog when not connected.
• Invalidate the copilot instructions cache on Google connect, disconnect, and stale-scope invalidation so routing updates mid-session (Composio handler already did this).
• composio-integration skill demoted for Gmail: fallback-only, GMAIL_FETCH_EMAILS example replaced with a Notion one. The gmail toolkit stays in CURATED_TOOLKITS for un-migrated users.