fix(agents): stop reporting an uninstall that removed nothing

[Adam-Dalloul]

Fixes the second half of #631: uninstall reports success even when it removed nothing and the agent is still installed.

Uninstall only ever removes the copy codeg manages. An OpenCode that came from ~/.local/bin, a package manager, or anywhere else on PATH survives it untouched and is still exactly what the next connection launches. Both surfaces claimed otherwise: the install stream always completed with <name> uninstalled successfully, and the settings panel forced the agent row to "not installed" and toasted "local version removed", even for an agent codeg had never installed.

Reproduction, from the log the reporter attached to the issue:

[ACP][OpenCode] No cached binary; using system C:\Users\...\.local\bin\opencode.exe from PATH

There is no managed tree for that agent, so clear_agent_cache takes its !dir.exists() early return, acp_uninstall_agent_core reports Ok, and the completion line says "uninstalled successfully". The next status read re-probes PATH and puts the version straight back, which is the "假成功" in the issue.

What changes:

• clear_agent_cache returns whether it actually removed a managed tree.
• acp_uninstall_agent_core re-probes after the removal and builds its completion line from what really happened, naming the version that survived.
• The agent settings panel reads the same re-probe instead of assuming, so the row keeps showing what is genuinely installed and the toast says an outside install is still in use.

Tests: uninstall_completion_message_reports_what_actually_happened pins all four outcomes and fails on the old unconditional message; survivingInstallVersion covers the probe result the panel branches on.

The first half of #631 (the binary cache "never lands where codeg reads it") is a separate matter and is not touched here. I could not reproduce it: the write path and the read path both derive cache_dir()/<agent>/<version>/<platform> from the same helpers, and the reporter's own log shows the cache genuinely empty rather than misplaced, because enabling an agent does not download anything. I left a note on the issue.