Improve Kimi Code runtime discovery

[ninjusai]

Summary

• probe Kimi Code's standalone ~/.kimi-code/bin install directory so GUI-launched Desktop processes can resolve kimi
• keep the Kimi preset on kimi acp and point its setup guidance to the official Kimi Code CLI documentation
• document the harness configuration and add regression coverage for discovery and preset arguments

Tests

• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml -- --check
• cargo test --manifest-path desktop/src-tauri/Cargo.toml kimi -- --test-threads=1 — 2 passed
• full serial Tauri suite — 1,693 passed, 10 ignored; the existing order-dependent managed_agents::runtime::tests::claude_spawn_uses_the_probed_cli_executable test failed in-suite and passed immediately in isolation on the same commit

[linxule]

One data point on the discovery path, in case it's useful for scoping this.

I hit this exact symptom (Kimi Code showing "CLI needed" on a GUI-launched Buzz while kimi worked fine in a terminal), but ~/.kimi-code/bin wouldn't have resolved it. Kimi Code also ships as an npm package (@moonshot-ai/kimi-code), so a bun add -g / npm-global install puts the executable in the package manager's bin dir instead:

$ ls ~/.kimi-code/bin
rg

$ command -v kimi
/Users/…/.bun/bin/kimi -> ../install/global/node_modules/@moonshot-ai/kimi-code/dist/main.mjs

~/.kimi-code exists and holds config/credentials/cache, but bin/ only has a vendored rg — no kimi. So the standalone-installer probe here and the package-manager install route are two separate cases.

The underlying cause is the same one this PR's comment describes, and it isn't Kimi-specific: ~/.bun/bin isn't in common_binary_paths(), and bun's installer writes its PATH export to ~/.zshrc (interactive), so find_via_login_shell() can't recover it either — same as the nvm situation already documented in resolve_command_uncached.

I opened #3343 to add ~/.bun/bin to common_binary_paths(), which should be complementary to this rather than overlapping: this PR covers the standalone installer, that one covers bun-installed CLIs generally (it fixed Codex on my machine too, for the same reason).