refactor: migrate self-update to the update-rs crate

[notheotherben]

Summary

Replaces the hand-rolled src/update/ self-update module with the external update-rs crate (0.4.0) — which was extracted from this project. The ~1,400-line module collapses to a small src/update.rs plus the dependency, with no change to the gt update UX or the Go-style release asset naming, and keeping Git-Tool's original update --state relaunch convention.

What changed

• Cargo.toml — update-rs = { version = "0.4", features = ["opentelemetry"] }. The shared human-errors 0.2.3, opentelemetry 0.32 and tracing-opentelemetry 0.33 all unify to single versions with the existing tracing-batteries stack, so update_rs::Error is our engine::Error (no conversion) and the global trace propagator is shared across the three update processes.
• src/update.rs — manager() builds a GitHubSource for SierraSoftworks/git-tool (naming::go("git-tool") → git-tool-<os>-<arch>[.exe], v tag prefix) and installs a GitToolLauncher that relaunches via gt update --state <json> (overriding update-rs's resume_args).
• commands/update.rs — the hidden --state arg drives resume (resume_from_arg); list/select/rollback go through the crate's Release/get_variant. The --list formatting is a pure format_release_list helper, and resume is covered by a network-free test (resume failures bubble up without the command reporting to Sentry).
• main.rs — no argv scanning. --update-resume-internal is kept as a tolerated hidden global so clap still accepts relaunches emitted by currently-installed releases (--update-resume-internal <s> --trace-context <c> update --state <s>); the update --state sub-command drives the resume.
• commands/open.rs — the passive "update available" check uses the new manager.

Telemetry & trace continuity

Trace-context propagation across the three update phases is handled natively by update-rs (the context rides inside the serialized update state), so no --trace-context argument is needed on the relaunch. update-rs emits its own #[instrument] spans for the update (its opentelemetry feature), which flow into the existing tracing → Honeycomb pipeline. The previous test-only GITHUB_TOKEN injection is no longer needed: production was already unauthenticated (no regression), and the update path's only mandatory-network test is now a pure unit test.

Verification

• cargo test --features pure-tests → 226 passed, 27 ignored; the new resume test and the list-formatting test are network-free.
• commands::open::tests::run (live update check) passes and correctly reports the latest release.
• gt update --list against real GitHub lists releases with correct markers.
• Relaunch handling smoke-tested both ways: gt update --state '{"phase":"no-update"}' exits 0, and the legacy --update-resume-internal <s> --trace-context <c> update --state <s> form (what installed v3.11 binaries emit) is tolerated and resumes via the sub-command — so in-flight cross-version updates keep working.
• cargo fmt --check clean; no new clippy warnings.

Recommended before release: a manual end-to-end three-phase update on each OS (e.g. gt update v3.11.0 to roll back, then gt update to roll forward) to exercise download → replace → cleanup and confirm no UAC prompt on Windows.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 82.89474% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.07%. Comparing base (3ae3187) to head (ee6f087).

Files with missing lines	Patch %	Lines
src/commands/update.rs	85.71%	7 Missing ⚠️
src/engine/http.rs	80.00%	3 Missing ⚠️
src/update.rs	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1885      +/-   ##
==========================================
+ Coverage   89.66%   90.07%   +0.40%     
==========================================
  Files         107      102       -5     
  Lines        9307     8653     -654     
==========================================
- Hits         8345     7794     -551     
+ Misses        962      859     -103     

Files with missing lines	Coverage Δ
src/commands/open.rs	95.12% <100.00%> (-0.12%)	⬇️
src/main.rs	42.46% <100.00%> (+2.20%)	⬆️
src/engine/http.rs	85.96% <80.00%> (-0.40%)	⬇️
src/update.rs	70.00% <70.00%> (ø)
src/commands/update.rs	89.69% <85.71%> (+4.63%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.