feat: add rulesync doctor command for config diagnostics

[dyoshikawa]

Summary

First slice of #2446: adds a read-only rulesync doctor command that diagnoses the configuration files (rulesync.jsonc / rulesync.local.jsonc) and reports problems grouped by severity (error / warning / info).

Closes #2446 (config-file check group; input-tree and workspace check groups are follow-ups as noted in the issue).

Why

Today the only feedback channel for a broken setup is rulesync generate. Because ConfigFileSchema is non-strict, a misspelled key such as "target" instead of "targets" is silently swallowed and generation quietly falls back to defaults. doctor gives a single, fast, read-only answer to "is my configuration correct?" — usable both as a first troubleshooting step and as a CI guard.

What it checks

• JSONC parse errors, with line/column
• Unknown or misspelled top-level keys, with "did you mean" suggestions derived from ConfigFileSchema (no hard-coded key list)
• Unknown tool targets / features (array and object forms), with nearest-name suggestions
• Deprecated ignore feature (warning)
• Object-form targets combined with features, including the case where the conflict only appears after merging base and local config files
• Conflicting target pairs (reuses the exported CONFLICTING_TARGET_PAIRS)
• $schema presence (info) and currency (warning)
• Structural violations on other keys via ConfigFileSchema.safeParse
• sources[].tokenEnv naming an unset environment variable (warning)
• inputRoot pointing at a nonexistent directory (error)

UX

• Exit code 1 when any error is present; --strict makes warnings fatal for CI
• Honors the global -j, --json flag (diagnostics + severity summary as structured data)
• -c, --config, -V, --verbose, -s, --silent wired like other commands
• Never writes files

Example

$ rulesync doctor
✖ rulesync.jsonc [config/unknown-key] Unknown key 'target'. It is silently ignored by 'rulesync generate'.
    ↳ Did you mean 'targets'?
✖ rulesync.jsonc [config/unknown-target] Unknown tool target 'cursorr' in the 'targets' object.
    ↳ Did you mean 'cursor'?
⚠ rulesync.jsonc [config/deprecated-feature] Feature 'ignore' in 'features' is deprecated.
    ↳ Use the 'permissions' feature instead.
Doctor found problems: 2 error(s), 1 warning(s), 0 info(s).

Tests

• Unit tests per check (40+ cases) plus command-level tests: clean-workspace happy path (exit 0, no diagnostics), broken fixtures per diagnostic code, --strict, --config, JSON capture, base+local merge conflict
• pnpm cicheck passes

🤖 Generated with Claude Code

[dyoshikawa]

@dyoshikawa Thank you!