fix(telemetry): make telemetry opt-in and anonymize install ID

[Patel230]

Problem

Phase 3 medium audit finding (C5): the CLI's detached telemetry:

• Used a hardware-derived machine ID (machineid.ProtectedID("trace-cli")) as the PostHog distinct_id, so events could be correlated to a physical machine across installs.
• Was opt-out only (TRACE_TELEMETRY_OPTOUT) — telemetry was ON by default for every user.

Fix

• Replace the hardware machine ID with anonymousID(): a random 128-bit value generated on first use and cached at os.UserConfigDir()/trace/telemetry-id (dir 0700, file 0600). Stable per install, not correlated to hardware.
• Make telemetry opt-in: only sent when TRACE_TELEMETRY_OPTIN=1 is set. The legacy TRACE_TELEMETRY_OPTOUT still force-disables regardless.
• Extract a sendDetached dispatch var so tests can assert dispatch behavior without spawning subprocesses.
• userConfigDir is now a var so tests isolate the ID file from the real user config dir (relevant on macOS where XDG_CONFIG_HOME is ignored).
• Drop the now-unused github.com/denisbrodbeck/machineid dependency.

Tests

• TestTrackCommandDetachedRespectsOptOut — opt-out wins over opt-in
• TestTrackCommandDetachedDisabledByDefault — no opt-in ⇒ no dispatch
• TestTrackCommandDetachedEnabledWithOptIn — opt-in ⇒ exactly 1 dispatch
• TestTrackPluginDetachedDisabledByDefault / TestTrackPluginDetachedEnabledWithOptIn
• Existing payload serialization tests unchanged

Verified: go build ./..., go vet ./..., go test ./... all green.