feat(db): warn at mount when TimescaleDB extension is missing

[mfreed]

History features (.history/, .log/, .savepoint/, .undo/) require TimescaleDB. The .build/ handler already refuses ',history' builds with a clear FSError when the extension is absent (fs/build.go:79-92), so users can't accidentally trigger raw SQL errors from CREATE TABLE WITH (tsdb.hypertable, ...).

What was missing: proactive notice at mount time. A user on vanilla PostgreSQL had no signal until they tried to opt into history.

This commit adds:

• Client.WarnIfTimescaleDBMissing(ctx): probes pg_extension and logs a single Warn-level line with an actionable hint when the extension is absent. Non-blocking; probe errors are swallowed rather than gating startup.
• Wired into all three mount entry points (fuse/mount_ops.go, fuse/fs.go, nfs/mount_darwin.go) immediately after the DB client is created.
• Two unit tests on the existing .build/ guard: absent ->
  FSError{ErrInvalidPath} with a hint mentioning TimescaleDB and CREATE EXTENSION; present -> guard falls through (no TimescaleDB-shaped error).

Surface coverage:

• Source-table CRUD: works on vanilla Postgres (no TimescaleDB).
• ',history' build: blocked at build-time with hint.
• .history/, .log/, .savepoint/, .undo/: unreachable unless the build succeeded, so the build-time guard is the only gate.
• Mount itself: never blocked; the warning is a heads-up.