fix(mcp): inherit project mcp overrides when creating new sessions

[JamesDAdams]

Summary

• Fix MCP session override inheritance: new sessions now properly inherit project-level disabled MCP servers and global MCP defaults.
• Centralize MCP override calculation in session-overrides.ts (computeDisabledServersForProject, initSessionMcpOverrides).
• Preserve disabled MCP servers when forking sessions (forkSession) and cleanup on delete (deleteSession).
• Fix ProjectSettingsModal payload to send empty overrides when dirty so settings can be cleared.

AI-Enhanced Development

Tell what models helped shape this PR:

• AI Models: Gemini 3.7 Flash

Cache Impact

Does this PR affect anything cached — system prompts, tool definitions, skills, or other context?

• Yes tool definitions and dynamic context now correctly reflect project MCP configuration on session start.

[co-l]

Merged, will be released in v2.0.136

AI-summary

• Rebased the PR onto latest develop and resolved 2 conflicts (session-create route and ProjectSettingsModal), keeping the newer buildSessionCreatedMessage broadcast and the mcpDirty clearing fix
• Verified the full unit suite (5108 passed), typecheck, lint, format and duplicate clean, plus MCP/session e2e green (36 tests)
• Confirmed the removed getAllSessionOverrides has no remaining references
• Pushed the rebased branch to the contributor's fork so the PR merges cleanly
• Squash-merged