fix(registry): suppress X-Nono-UUID when update check is opted out

[scp7]

Summary

Fixes #1507.

Registry requests to registry.nono.sh continued to send the persistent installation identifier X-Nono-UUID even after the user opted out of the update check, whenever the update-check state file already existed on disk (the default after the first nono run). The opt-out (NONO_NO_UPDATE_CHECK / [updates] check = false) gated only the update.nono.sh ping, not the registry header, so the documented opt-out was inaccurate in the common case.

Approach

• Added a single authoritative opt-out predicate update_check_opted_out() in crates/nono-cli/src/update_check.rs (env NONO_NO_UPDATE_CHECK presence — any value — or config [updates] check = false).
• Routed start_background_check() through it (behaviour-preserving).
• Gated read_installation_uuid() on the predicate — it now returns None when opted out, suppressing X-Nono-UUID at the source for every consumer, even if a state file was written on an earlier run. The state file is intentionally not deleted (transmission stops; users still remove the file to erase the on-disk UUID).
• De-duplicated pack_update_hint::is_opted_out() to reuse the shared predicate.
• Corrected the header doc comment in registry_client.rs.

The coarse fingerprinting headers (X-Nono-Platform/Arch/CI/Install-Source) are intentionally out of scope — they remain gated by the separately-tracked master telemetry switch. This change is limited to the persistent identifier.

Tests

• update_check::test_read_installation_uuid_respects_opt_out — reader returns the UUID when opted in; None under both the env-var and config opt-outs; state file left on disk.
• registry_client::context_headers_gate_uuid_on_opt_out — X-Nono-UUID present when opted in, absent when opted out, while X-Nono-Platform/X-Nono-Install-Source remain (pins the scope boundary).

Both tests are hermetic (serialized on ENV_LOCK, all env vars guard-managed, isolated temp HOME/XDG_STATE_HOME/XDG_CONFIG_HOME). make ci is clean locally.

Files consulted

crates/nono-cli/src/update_check.rs, crates/nono-cli/src/registry_client.rs, crates/nono-cli/src/pack_update_hint.rs, crates/nono-cli/src/config/user.rs, crates/nono-cli/src/state_paths.rs, crates/nono-cli/src/config/mod.rs, crates/nono-cli/src/test_env.rs.

Contributor disclosure

This PR was prepared by an AI-assisted contributor at the request of the maintainer, following the repository Coding Agent Contribution Policy (issue filed and disclosed before any code change).

Agent Compliance Check

• I am not prohibited from contributing under this policy
• An issue already exists (Registry requests still send X-Nono-UUID after update-check opt-out #1507)
• I described my intent and approach in the issue discussion
• I reviewed repository coding and security rules for the affected area
• I provided required attribution for reused or adapted code
• I did not use forbidden unwrap/expect in production code (test code uses .expect() consistent with the repo's existing test convention; clippy::unwrap_used passes under make ci)
• I used NonoError where required
• I validated and canonicalized all relevant paths
• This PR matches the approved or disclosed issue scope

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[github-actions]

PR Review Summary

Size

Metric	Value
Lines added	+167
Lines removed	-19
Total changed	186
Classification	Medium (50–300 lines)

Affected crates

• crates/nono-cli — CLI changes. Verify argument parsing, flag documentation, and UX behaviour across supported platforms.

Blast radius — Contained

This PR touches: source code

---

_{Updated automatically on each push to this PR.}

[nogent-nolabs-ai]

nogent code review

No blocking issues; 0 suggestions.

Findings: none flagged in scope.

_{Automated code + security review. CI already covers clippy, rustfmt, tests, cargo-audit and commit-lint.}

[SequeI]

lgtm!