feat: doctor — detect missing hook binaries on PATH

[vishnujayvel]

Problem

When hooks reference binaries that aren't on PATH, they fail silently on every event. The user gets no feedback — just mysterious slowness as "command not found" errors are swallowed or cause retries.

From the incident: 7 hooks called hookwise dispatch but the binary wasn't on PATH. Each silently failed on every matching event, compounding the slowness with no visible errors.

Requirements

F1.3 — Binary Existence Check

For each hook in Claude Code settings, extract the executable name and verify it exists on PATH using exec.LookPath:

FAIL  hook-binary: "hookwise" not found on PATH (used by 7 hooks)
       → These hooks will fail silently on every event.
       → Fix: ensure hookwise is installed and on PATH, or remove these hooks.

PASS  hook-binary: "custom-lint.sh" found at ./scripts/custom-lint.sh

Implementation Notes

• Read hooks from ~/.claude/settings.json and ~/.claude/projects/*/settings.local.json
• Extract first token of each hook command as the binary name
• Use exec.LookPath for PATH resolution
• For relative paths (e.g., ./scripts/foo.sh), check existence from project directory
• Report per-binary, not per-hook (group hooks that use the same binary)

Acceptance Criteria

• Doctor reads Claude Code settings files and extracts hook commands
• Each hook's binary is verified via exec.LookPath
• Missing binaries produce FAIL output with count of affected hooks
• Found binaries produce PASS output with resolved path
• Gracefully handles: no settings files, empty hook arrays, complex commands (pipes, &&)
• Unit test with mock settings file containing missing and present binaries

Context

This is P0 — catches the #1 silent failure mode from the incident.

📎 Design doc: docs/plans/2026-03-07-hook-safety-and-onboarding-design.md (Feature 1.3)

[vishnujayvel]

Implemented in #75 (hook-safety suite, merged to main as 801113f).