✨ feat: Add anonymous PostHog telemetry to mb CLI

[bdougie]

Summary

• Adds anonymous telemetry via PostHog to track installs, DAU, and command usage
• New pkg/telemetry package with UUID persistence (~/.mb/telemetry.json) and PostHog client wrapper
• Captures events: mb_cli_installed, mb_cli_command_run, mb_cli_sandbox_created, mb_cli_sandbox_stopped, mb_cli_ssh_connected, mb_cli_mixtape_pulled, mb_cli_error
• Opt-out via --disable-telemetry flag or automatic CI environment detection
• No PII collected -- anonymous UUID, OS, arch, command name, success/failure only

Design decisions

• Nil-safe receiver pattern -- NewPosthogClient returns nil when disabled, all capture methods are no-ops on nil receiver. No PostHog connection or UUID file created when telemetry is off.
• CI auto-detection -- Checks CI, GITHUB_ACTIONS, GITLAB_CI, CIRCLECI, TRAVIS, JENKINS_URL, BUILDKITE, CODEBUILD_BUILD_ID
• Command tracking in subcommands -- CaptureCommandRun(cmd.CommandPath()) called in each subcommand's RunE so events log "mb up", "mb ssh" etc. instead of just "mb"
• SSH flush before exec -- telem.Done() called before ssh.ExecSSH() since syscall.Exec replaces the process and PersistentPostRunE never runs
• Home dir at call time -- Uses os.UserHomeDir() at call time instead of os.Getenv("HOME") at package init for container robustness
• Tapes consistency -- Disable mechanisms match tapes PR #149 (flag + CI detection, no custom env var)

Test plan

• 11 Ginkgo tests pass (UUID persistence, CI detection, context round-trip, nil safety, invalid JSON recovery)
• go vet ./... clean
• Full binary builds cleanly
• Run mb up and verify event appears in PostHog
• Run with --disable-telemetry and confirm no events sent
• Verify ~/.mb/telemetry.json is created on first run
• Confirm second run reuses existing UUID (not first-run)

[bdougie]

Applied the review feedback from papercomputeco/tapes#149 to align this PR:

• PostHog API key is now a build-time ldflag, no longer hardcoded in source. Defaults to empty so dev builds have telemetry disabled. The key should be injected as a CI secret (POSTHOG_API_KEY).
• Dagger BuildRelease, ReleaseLatest, and ReleaseNightly accept an optional postHogPublicKey parameter, threaded through the pipeline.
• Makefile adds POSTHOG_API_KEY ?= and conditionally passes it to both local ldflags and dagger calls.
• disable-telemetry flag bound to Viper via BindPFlag, so config file / MB_DISABLE_TELEMETRY env / --disable-telemetry flag all work with proper precedence. Removed the out-of-band env var check.
• initTelemetry calls mbconfig.Init first, then checks viper.GetBool("disable-telemetry") instead of manually reading the flag.