feat(agents): apply MCP preferences on reload and show TLDR freshness #904

Changing a global built-in MCP setting currently affects only newly created agents: existing agents reload their saved capability list. Global settings should apply on the next provider start/reload while explicit per-agent choices remain durable.

The TLDR peek also needs small freshness indicators so the user can distinguish a recently active agent from an old status note.

Acceptance criteria:

• All configurable built-in MCP defaults apply to new agents and existing agents on their next actual provider reload/restart; running agents are not interrupted by Settings changes.
• Separate explicit per-domain overrides from the active MCP snapshot. Keep provider restrictions and explicit all-off creation requests. Provide a route to return an agent to global settings.
• Migrate legacy enabled domains conservatively; absent legacy domains inherit the global preference because historical snapshots did not record why a domain was absent.
• Cover individual/bulk reload, provider transitions, undo and restart/wake spawning, without misreporting capabilities when adopting a live backend.
• TLDR-enabled starts use the existing managed-skills authority before provider launch.
• TLDR overlays show small Last active and Note written indicators for the displayed agent, derived independently from activity evidence and the stored TLDR timestamp. Unknown activity must not be fabricated.
• Recent timestamps use readable relative text; after 30 days use a local calendar date with exact time available accessibly. Time labels update while the preview is open without background polling when hidden.
• Preserve summary continuity, centered content, mounted feeds/terminals and pane-local placement.
• Add meaningful regression coverage at real action/launch and renderer boundaries; verify type checking, keybindings and CI.