telemetry: document env var, add --no-telemetry flag

[staylor]

Problem

LIGHTPANDA_DISABLE_TELEMETRY already opts out of telemetry, but:

• It is missing from lightpanda --help (any subcommand).
• The README only mentions it in one sentence and shows it as =true, which suggests the value matters — it does not. The current implementation uses std.process.hasEnvVarConstant, so the variable just needs to be present (any value, including empty).
• There is no CLI equivalent, so users who do not want to set environment variables (CI configs, one-off invocations, MCP launchers) have no way to opt out.

Changes

• Add --no-telemetry as a common option for serve, fetch, and mcp. Wired into the existing CommonOptions block in Config.zig so it picks up the standard --kebab-case / --snake_case handling and unknown-flag validation for free.
• Make the crash reporter honor the flag. crash_handler.zig calls telemetry.isDisabled() from a panic context where the App/Config are not reachable, so I added a module-level cli_disabled flag + setDisabledByCli setter that App.init populates from config.noTelemetry() before Telemetry.init runs. This guarantees --no-telemetry suppresses crash reports too.
• Document both opt-outs. src/help.zon now lists --no-telemetry under common options and references the env var. The README clarifies that the env var only needs to be present (no specific value required) and that debug builds always disable telemetry.
• Extend the existing telemetry test to assert that the CLI override path also short-circuits isDisabled().

Verification

zig fmt --check ./*.zig ./**/*.zig    # clean
make test                              # 720 of 720 tests passed

./zig-out/bin/lightpanda help fetch | grep -A4 no-telemetry
  --no-telemetry
          Disable anonymous usage telemetry and crash reporting.
          Equivalent to setting the LIGHTPANDA_DISABLE_TELEMETRY
          environment variable (any value, including empty).
          See https://lightpanda.io/privacy-policy for details.

./zig-out/bin/lightpanda fetch http://example.com --no-telemetryX
FATAL app : unknown argument
      mode = fetch
      arg = --no-telemetryX

Notes

• No existing behavior changes when neither the flag nor env var is set.
• The semantics of LIGHTPANDA_DISABLE_TELEMETRY are unchanged; only its documentation is updated to match the actual implementation.
• Debug/test builds remain hard-disabled, matching the prior invariant covered by telemetry: always disabled in debug builds.

[github-actions]

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[karlseguin]

I do like that this is more discoverable. My only reservation is that there's a window (before App.init is called) where the flag is ignored. So it would be possible for --no-telemetry to be set but a crash report would still happen. I assume this is likely common. Punting this one to @krichprollsch also.

[krichprollsch]

Thanks for the contribution. After discussion and while we understand your points, as an open source project, telemetry is vital to sustaining development in the long run. That's why we prefer to close this change and keep only the env var for now.

There is no CLI equivalent, so users who do not want to set environment variables (CI configs, one-off invocations, MCP launchers) have no way to opt out.

For CI config it's common to setup env vars, we do that ourself.

On-off invocation is also quite easy to do: LIGHTPANDA_DISABLE_TELEMETRY=true lightpanda

And in mcp configuration, for example, with Claude you can setup env vars easily.

So IMO the env var is well supported and good enough in all these cases.