#3572 agent-runner and deno-runner: support Windows (both refuse to start: Unsupported operating system)

Problem

@swamp/agent-runner and @swamp/deno-runner can't run on Windows, even though swamp itself supports Windows (#3475). Both download and manage their own binaries, and every method calls a detectPlatform() that only knows darwin and linux. On Windows it throws Unsupported operating system: windows before any download or spawn.

Neither manifest says so (platforms: []), so a Windows user only finds out when a method throws. The reporter of #3570 filed from Windows (Git Bash) while using @swamp/agent-runner.

Where it fails

Allowing windows in detectPlatform() isn't enough. Each runner assumes a Unix system all the way through:

@swamp/agent-runner

• _lib/platform.ts detectPlatform(): only darwin/linux. The Platform type in _lib/types.ts has only the linux-* and darwin-* keys.
• _lib/providers/claude.ts: downloads <version>/<platform>/claude and caches it as claude; a Windows build would be claude.exe under a Windows platform key. What Anthropic publishes for Windows needs checking.
• _lib/providers/codex.ts: fetches @openai/codex-<platform> from npm, extracts it with tar, searches for a file named exactly codex, then runs chmod 0o755.
• _lib/runner.ts: spawns with clearEnv: true and a Unix-only allowlist (HOME, TMPDIR, SHELL, XDG_*, ...). Windows processes generally need SystemRoot, USERPROFILE, APPDATA, LOCALAPPDATA, TEMP/TMP, PATHEXT and COMSPEC. Without them, auth=cli likely can't find the Claude CLI's stored login.
• Claude outputInstructions tells the agent to write its result with a tee heredoc, which needs a POSIX shell.

@swamp/deno-runner

• _lib/installer.ts detectPlatform(): the same darwin/linux-only check. archiveFilename() maps only the four Unix release zips; Deno publishes deno-x86_64-pc-windows-msvc.zip.
• extractDenoFromZip shells out to unzip, which isn't on a stock Windows. It expects a bare deno file (Windows ships deno.exe), then runs chmod 0o755.
• _lib/executor.ts: the same Unix-only environment allowlist when inheritEnv is false.

Proposed

Add Windows (at least windows-x64) to both runners:

• Platform keys and download names that match each upstream's Windows release (with the .exe suffix).
• Extraction that doesn't depend on Unix tools: unpack in-process, or use what Windows ships.
• Skip chmod on Windows.
• A Windows set of forwarded environment variables, keeping the cleared-environment design.
• For agent-runner, output instructions that work without a POSIX shell, or a check that one is available.

Until that lands, a cheap step is to state the supported platforms in both manifests and READMEs, so the limit is visible before a method throws.

Testing

Unit tests for the Windows platform key, download name and environment allowlist (no live downloads, following the existing installer and provider tests). Plus a manual run of run and review (agent-runner) and of the deno-runner methods on a Windows host.