Add Clarity and an OpenAPI agent surface

[sarthakagrawal927]

Two commits, both previously uncommitted in the local checkout:

1. Microsoft Clarity alongside PostHog, with www.clarity.ms allowed in
   the CSP script-src.
2. Agent surfaces — /openapi.json, advertised from the agent catalog and
   llms.txt.

Deliberately not included: the in-progress Better Auth refactor sitting in
the same working tree (auth → lazy getAuth(), db-schema.sql,
migrations/0002_better_auth_tables.sql, and new tests for auth and
user-menu). That is a separate change with its own review surface and is
still uncommitted locally.

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