fix(windows): silence tirith-unavailable banner + skip install/spawn attempts on unsupported platforms (#26718)

Tirith ships no Windows binary, so on every Windows CLI startup users
saw a scary 'tirith security scanner enabled but not available' banner
they could not act on. The banner suggested degraded security; in
reality pattern-matching guards still run and the message was pure noise.

Fix:
- New public is_platform_supported() helper in tools/tirith_security.py
  that returns False when _detect_target() doesn't resolve (Windows, any
  non-x86_64/aarch64 arch).
- ensure_installed(), _resolve_tirith_path(), and check_command_security()
  short-circuit on unsupported platforms: cache _resolved_path =
  _INSTALL_FAILED with reason 'unsupported_platform', skip PATH probes,
  skip the background download thread, skip the disk failure marker, and
  return allow with an empty summary from check_command_security so the
  spawn loop never fires.
- Explicit user-configured tirith_path is still honored everywhere (a
  user who built tirith themselves under WSL keeps that path).
- CLI banner in cli.py gated on is_platform_supported() — fires only on
  platforms where tirith *should* work but isn't installed.
- Docs note tirith's supported-platform list and point Windows users at
  WSL.

Tests: tests/tools/test_tirith_security.py +8 tests covering Linux
x86_64, Darwin arm64, Windows, and unknown-arch verdicts plus the
silent ensure_installed / check_command_security / _resolve_tirith_path
fast-paths and the explicit-path override.

  test_tirith_security.py     75 passed (8 new + 67 pre-existing)
  test_command_guards.py      19 passed

website/docs/user-guide/security.md

@@ -537,6 +537,8 @@ security:
 
 When `tirith_fail_open` is `true` (default), commands proceed if tirith is not installed or times out. Set to `false` in high-security environments to block commands when tirith is unavailable.
 
+Tirith ships prebuilt binaries for Linux (x86_64 / aarch64) and macOS (x86_64 / arm64). On platforms with no prebuilt binary (Windows, etc.), tirith is silently skipped — pattern-matching guards still run, and the CLI does not surface an "unavailable" banner. To use tirith on Windows, run Hermes under WSL.
+
 Tirith's verdict integrates with the approval flow: safe commands pass through, while both suspicious and blocked commands trigger user approval with the full tirith findings (severity, title, description, safer alternatives). Users can approve or deny — the default choice is deny to keep unattended scenarios secure.
 
 ### Context File Injection Protection