[Bug] prime-agent update on stable or beta never shows the version it installed

[Hotragn]

Summary

prime-agent update on the stable or beta channel never prints the version it installed. It always says "updated to the latest build", and /update in the TUI shows "the latest build".

Reproduction

1. Install a stable release, for example 0.10.2.
2. Run prime-agent update.
3. It prints "updated to the latest build — restart prime-agent to run the new build" instead of naming the version.

Cause

run_installer_from in crates/pa-core/src/update/installer.rs reads the new version only through launcher_version. That accepts a --version answer only if it has a -continuous.<sha> stamp, so that the old TS launcher never matches.

Real channel builds don't have that stamp. Stable builds are assembled with --version and no sha (release.yml), and restamp_continuous.py moves beta builds to the bare release version. So launcher_version returns None on every stable or beta update.

Proposed fix

When the probe returns nothing, fall back to the version line in the install marker the script just published (installed_version(prefix)). run_bundled_installer already reads the version that way. Patch below.

[Hotragn]

Patch: 576ffe3 on fix/update-reports-stable-version.

It adds a STABLE_INSTALLER mock next to MOCK_INSTALLER. The mock writes a launcher that answers a bare 0.10.0 and a marker that records it. The new test a_stable_update_reports_the_installed_version gets None before the fix and Some("0.10.0") after it. The existing continuous-stamp tests still pass, and fmt and clippy are clean.