fix: prompt to re-run after self-update instead of continuing on stale binary

[uribrecher]

Problem

Running thicket start when a new release is available prompts to update; after agreeing, the update succeeds but the start page still shows the previous version. You have to stop the run and start again to actually use the new version — confusing UX.

Root cause

The self-updater (updater.Apply → swapBinary) atomically replaces the binary file on disk, but the running process keeps executing the old code already loaded in memory — replacing an executable's file doesn't hot-swap a live process. So after the swap, PersistentPreRun returns and the start command body runs the old version's UI. The old code even printed "Continuing with your original command using the previous binary…", which is exactly the stale render the user saw.

Fix

After a successful in-place update, print a completion message and exit cleanly instead of continuing on the stale binary:

Update to v0.10.0 completed successfully. Please re-run the CLI to use this latest version.

The next invocation runs the freshly-installed binary, and the 24h-cached update probe then sees it's already on latest — so nothing re-prompts. No new flags needed; it's self-correcting.

Changes

• internal/updater/updater.go — extract finishSuccessfulUpdate(errOut, tag); print the re-run message and exit(0). exit is an injectable package var (= os.Exit) following the package's existing test-seam pattern.
• internal/updater/updater_test.go — TDD test asserting the message content and a single exit(0).
• README.md — document the re-run behavior in the Self-update section.
• .changes/unreleased/ — changie Fixed fragment.

Verification

• go test ./... — full suite green
• go build ./... — builds
• golangci-lint run + go vet — clean

🤖 Generated with Claude Code

https://claude.ai/code/session_01BBqhECkiepRLjoGvg2fG2w

[Copilot]

Pull request overview

This PR fixes a confusing self-update UX where thicket start (and other commands) could continue executing the old in-memory binary after an on-disk self-update, causing the UI to show the previous version. After a successful in-place update from the pre-run prompt, the CLI now prints a clear completion message and exits cleanly so the user can re-run on the newly installed binary.

Changes:

• Add an injectable exit test seam and a finishSuccessfulUpdate helper that prints a re-run message and exits with code 0.
• Update updater tests to assert the message content and that the process exits exactly once with code 0.
• Document the “re-run after successful self-update” behavior in the README and add a changelog fragment.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
README.md	Documents that thicket exits after a successful self-update and prompts the user to re-run to load the new binary.
internal/updater/updater.go	Stops post-update execution on the stale binary by printing a completion message and exiting after a successful Apply.
internal/updater/updater_test.go	Adds a unit test verifying the re-run message and the single exit(0) behavior.
.changes/unreleased/Fixed-20260623-073650.yaml	Records the behavioral change in the changelog fragments.

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