fix(telemetry): keep the agent id per machine, not per project folder

[AnderRV]

Why

The anonymous agent id sent as X-ZR-Agent-Id on signup was stored in .zenrows/config.json, inside the project folder. Every new folder minted a new id, so one machine looked like a new device in each project, and a retry from a machine that had already signed up (for example after a rate limit) could not be tied back to it.

Change

• getOrCreateTelemetryId() stores the id once per machine, in telemetry-id under:
  • ZENROWS_CONFIG_HOME if set (tests, sandboxes)
  • %APPDATA%\zenrows on Windows
  • $XDG_CONFIG_HOME/zenrows, else ~/.config/zenrows
• An id already in a workspace config is adopted as the machine id on first run, so existing installs keep their identity.
• If the machine directory can't be written (read-only home, locked-down sandbox), it falls back to the workspace config as before.
• Still a random UUID, still suppressed by ZENROWS_TELEMETRY=off / telemetry: "off".
• The test bootstrap points ZENROWS_CONFIG_HOME at a temp dir so the suite never writes to the real home directory.

Verification

• npm run typecheck: clean.

• npm test: 276/276 pass.

• New tests/telemetry-id.test.ts:

  • two workspaces on one machine send the same id
  • an existing workspace id is adopted
  • an unwritable machine directory falls back to the workspace

  The first two fail against the previous implementation.

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