fix(spawn): refuse missing harness binaries before launch

[Amplify-Logic]

Summary

• Fail closed in fm-spawn when a verified harness launch binary is missing or fails a cheap --version probe, before any pane/endpoint or task meta is created.
• Resolve the binary through the shared backend executable resolver (same path as bootstrap) and surface a one-line refusal with the install hint.
• Raw launch-command spawns stay exempt (documented in the script header): arbitrary shell syntax cannot be probed safely without executing the unverified command.
• Colocated coverage in tests/fm-spawn-launch-preflight.test.sh for missing-binary refusal, healthy six-adapter spawn, raw-command exemption, and probe timeout/kill behavior.

Test plan

• bash tests/fm-spawn-launch-preflight.test.sh (all cases passed locally)
• Confirm healthy spawns still work for claude, codex, opencode, pi, grok, and cursor when their CLIs are present
• Confirm a deliberate missing-CLI spawn refuses loudly with the install hint and leaves no dead pane