fix(desktop): clean ANSI escapes from agent log tail + drop dead AppHandle plumbing

[wesbillman]

What

Two related fixes for the desktop agent log viewer:

1. Strip ANSI escapes from the agent log tail. The harness (sprout-acp) emits colorized tracing output. When its stdout/stderr is redirected to a log file by the desktop spawn path, the escape codes get baked into the file and render as ESC[2m-style gunk in the desktop log viewer's <pre>.
2. Drop unused AppHandle plumbing in command discovery. While in the area, removed a dead app: Option<&AppHandle> parameter that was threaded through five functions in managed_agents/discovery.rs and never read at any layer. The unused-variable warning that surfaced this is now silenced.

How

• read_log_tail in desktop/src-tauri/src/managed_agents/storage.rs pipes the buffer through strip-ansi-escapes::strip_str before returning. The crate delegates to vte — the same terminal parser Alacritty uses — so it correctly handles CSI, OSC, DCS, and C1 codes that a hand-rolled scrubber would silently mangle.
• sprout-acp itself is not changed. Terminals, tail -f, CI logs, etc. still get the colors.
• app: Option<&AppHandle> is removed from command_search_dirs, resolve_workspace_command, resolve_command, resolve_command_uncached, and command_availability. All six call sites (agent_discovery, agent_models, media, runtime) updated. The discover_managed_agent_prereqs Tauri command no longer needs the injected AppHandle either. Frontend invoke contract unchanged.

Why

• ANSI parsing is deceptively non-trivial — a 50-line hand-rolled stripper plus tests would still miss DCS/C1 cases. strip-ansi-escapes is one line, well-maintained, and the standard Rust answer (~70M downloads). Net new transitive dep is just vte.
• The dead AppHandle parameter was carrying ghost intent across the public API. If a future change needs AppHandle (e.g. resolve_resource for bundled sidecars), threading it back in will be a self-documenting diff rather than dead plumbing.

Tests

• New unit test in storage.rs covers a typical colorized tracing line.
• cargo check --no-default-features clean with no warnings.
• Lefthook pre-push (rust-fmt, rust-clippy, rust-tests, web-check, desktop-check, desktop-tauri-fmt, desktop-tauri-check, desktop-build, mobile-check, mobile-test) all green.

[wesbillman]

BEFORE:

�[2m2026-05-27T00:22:29.381046Z�[0m �[32m INFO�[0m �[2msprout_acp�[0m�[2m:�[0m sprout-acp stopped

AFTER:

2026-05-27T15:37:15.552965Z  INFO sprout_acp: relay observer enabled