fix(herdr): fall back from stale executable paths

[osolmaz]

Opened on behalf of Onur Solmaz (osolmaz).

Summary

Herdr link clicks could stop working when the plugin kept an executable path that no longer existed.
Ghzinga now tries that exact path first and falls back to herdr from PATH only when the configured command cannot start because it is not found.
This keeps link clicks working without changing or restarting Herdr, and it prevents mutating commands from running twice.

What Changed

The Herdr plugin now uses one command runner for pane checks, focus, and pane creation.
The fallback keeps the existing plugin environment and has a narrow retry rule.

• Retry only after a NotFound process spawn error.
• Do not retry after the command starts, returns a nonzero status, or has another spawn error.
• Pass the configured path unchanged, including a literal (deleted) suffix.
• Document the behavior and add focused unit and shell coverage.
• Update the isolated live smoke check to use the current Overview, Commits, and Files tabs.

Testing

The full local gate passed, including tests, coverage, audit, mutation testing, install checks, Slophammer, SimpleDoc, and capture validation.
The Herdr integration also passed in a disposable named session.

• scripts/ci-local.sh
• plugins/herdr/test/test-open.sh
• plugins/herdr/test/test-viewer.sh
• HERDR_PLUGIN_LIVE_SESSION=gzg-fallback-0906-2 scripts/herdr-plugin-live-smoke.sh

Risks

The change is limited to process startup failures for Ghzinga-owned Herdr plugin commands.
A command that starts is never retried, so existing command failure behavior remains unchanged.

[osolmaz]

Final verification report:

• scripts/ci-local.sh passed, including 87.25% line coverage, cargo audit, 16 mutation tests (13 caught and 3 unviable), Slophammer, install checks, shell tests, SimpleDoc, and capture validation.
• The isolated Herdr live smoke passed in named session gzg-fallback-0906-2; it opened and reused the Ghzinga plugin pane without touching the active default session.
• Pi Reviewer found no issues and rated the change correct with 93% confidence.
• GitHub Actions verify passed.