Problem

On Windows, Buzz Desktop discovers ACP harnesses by resolving their command
on the Windows PATH. Several supported harnesses are commonly installed
inside WSL, not on the Windows host — Hermes Agent (hermes-acp) is the
canonical case: it is a Linux-first install, so on a Windows machine running
Hermes in WSL, Doctor reports Not installed and the harness cannot be
started, even though it works perfectly inside the distro.

There was no WSL story anywhere in the harness discovery/spawn path (the only
WSL awareness in the tree is is_windows_apps_alias, which exists to avoid
WindowsApps stub launchers, issue #2328).

What this PR does

Buzz now falls back to the default WSL distribution when a preset or
custom harness command is missing from the Windows PATH:

1. Discovery (managed_agents/wsl.rs, new) — probes the default distro
   via wsl.exe -e sh -c …, checking ~/.local/bin, ~/bin,
   /usr/local/bin, then the distro PATH (command -v). Positive results
   mark the harness Available with a wsl://… binary path in Doctor.
   Resolutions are cached for the app lifetime (mirrors resolve_command's
   cache contract). Only bare command names are ever probed — paths are
   rejected up front (no injection surface, no nonsense lookups).

2. Spawn contract (runtime.rs → buzz-acp) — when the host PATH
   resolution failed but the WSL probe succeeded, Desktop sets
   BUZZ_ACP_AGENT_WSL_PATH (absolute in-distro path) and optionally
   BUZZ_ACP_AGENT_WSL_DISTRO on the buzz-acp child.
   BUZZ_ACP_AGENT_COMMAND keeps the original bare identity
   (hermes-acp), so all per-runtime defaults keyed on the command name
   (default_agent_env, default_agent_args) apply unchanged.

3. Spawn wrapping (buzz-acp/src/acp.rs) — AcpClient::spawn reads the
   contract and launches wsl.exe [-d distro] -e <linux_path> <args> instead
   of the bare command. WSL interop bridges the stdio pipes; the ACP/NDJSON
   channel is byte-transparent. Contract values are validated (absolute POSIX
   path required); a set contract on a non-Windows host is ignored with a
   warning.

4. Environment forwarding — wsl.exe children only import Windows-side
   variables listed in WSLENV. buzz-acp builds the forward list from
   every agent-intended key (per-runtime defaults like
   HERMES_ACP_SKIP_CONFIGURED_MCP=1, persona env like GOOSE_PROVIDER,
   merged CODEX_CONFIG), merging into any pre-existing WSLENV without
   duplicating flagged entries (GOPATH/p style). Without this, agents in
   WSL silently lose their env — for Hermes specifically this re-introduces
   the [Bug] Hermes model discovery times out in Buzz Desktop v0.5.0 #3355 startup-stall the skip flag exists to prevent.

Verification (real, on a Windows 11 + WSL2 machine)

Environment: Windows 11, WSL2 Ubuntu, Hermes Agent v0.19.0 at
/home/userspace/.local/bin/hermes-acp, Buzz Desktop installed at
C:\Users\user1\AppData\Local\Buzz\.

1. Baseline bug: hermes-acp absent from Windows PATH → Doctor shows
   Hermes Not installed.
2. Stdio transparency: spawning
   C:\Windows\System32\wsl.exe -e /home/userspace/.local/bin/hermes-acp from a
   Windows parent with piped stdio returns a byte-clean ACP initialize
   response (agentInfo: hermes-agent 0.19.0, protocolVersion: 1).
3. Env forwarding: without WSLENV, hermes-acp hangs before
   initialize (the [Bug] Hermes model discovery times out in Buzz Desktop v0.5.0 #3355 MCP-startup stall). With
   HERMES_ACP_SKIP_CONFIGURED_MCP=1 set Windows-side and forwarded via
   WSLENV, initialize answers immediately.
4. Zero-config workaround validated: the manual custom-harness JSON
   documented in docs/windows-wsl-harnesses.md (same mechanism this PR
   automates) loads and works on the current released build.

Tests

• buzz-acp: 8 new unit tests (build_wsl_wrap validation, prefix_args,
  merge_wslenv order/dup/flag handling). cargo test -p buzz-acp: 648
  passed, 0 failed (full suite, Linux). cargo clippy -p buzz-acp --all-targets: clean.
• Desktop wsl.rs: 6 unit tests (probe script contract pins ~/.local/bin
  before PATH, no login shell; stdout parsing; display path;
  probeable-command validation incl. metacharacter rejection; negative-result
  caching) — pass on Linux. Process-spawning probes are cfg(windows)-gated;
  the Windows-only code paths (resolve_wsl_exe, probe spawn) were
  compile-verified for x86_64-pc-windows-msvc.
• cargo check (Linux) on desktop/src-tauri: green, zero warnings.
  cargo test managed_agents:: on desktop/src-tauri: 906 passed, 0
  failed (includes the 6 new wsl.rs tests + all pre-existing discovery,
  custom-harness, spawn-hash, and readiness tests).
• Full x86_64-pc-windows-msvc workspace check was not runnable in the dev
  container (crypto -sys crates need a Windows C toolchain); Windows CI
  covers it. Every Windows-gated line added by this PR is either
  std-only Rust (verified via the standalone msvc-target compile described
  above) or mirrors existing patterns in git_bash.rs.
• Existing discovery tests bind the resolver seam (preset_catalog_entry) —
  unchanged, still green.

Notes for reviewers

• Scope: preset + custom harnesses only. Tier-1 builtins (auth probes,
  install scripts) still assume host-native installs — a follow-up could
  extend the same cached-resolution mechanism to their probes.
• Perf: probes run once per missing command per app launch, cache-first
  on both the discovery and spawn paths. WSL-unavailable machines
  short-circuit on wsl.exe resolution.
• Future: WslCommandResolution.distro is plumbed through the whole
  contract but always None today (default distro only) — multi-distro
  discovery needs no contract change.
• New public API: none (all pub(crate) / private). Docs added:
  docs/windows-wsl-harnesses.md.