fix(mcp): create_room advertises default budgets 60/24; backend applies 200/168

[vybe]

Summary

The create_room MCP tool advertises default budgets of 60 messages / 24 hours, but the backend applies 200 messages / 168 hours when the caller omits them. An MCP client reading the tool schema is told the wrong defaults.

Context

Found during the September user-docs sweep (PR #2746). collaboration/rooms.md was written to the backend values.

• src/mcp-server/src/tools/rooms.ts ~line 111: max_messages … describe("Message budget (default 60)")
• src/mcp-server/src/tools/rooms.ts ~line 113: ttl_hours … describe("Auto-close after N hours (default 24; 0 = never)")
• src/backend/shared_sessions/service.py: DEFAULT_MAX_MESSAGES = 200, DEFAULT_TTL_HOURS = 168

The admin can also change the defaults under Settings → Retention → Room budgets, so the tool description should not hard-code a number at all, or should say "the platform default (200 / 168 h unless an admin changed it)".

Acceptance Criteria

• create_room's parameter descriptions no longer state 60 / 24.
• The descriptions either name the real fallback (200 messages / 168 hours) or say the platform default applies and point at the Room budgets setting.
• A test pins the tool description against the backend constants, or the description is phrased so it cannot drift again.

Technical Notes

Description-only change in the MCP server; no backend change.