feat: auto-wire ctx hooks into .claude/settings.json on harness init

[saldestechnology]

What

ctx harness init --mode local now merges the ctx hooks + permissions into .claude/settings.json itself, instead of only printing a snippet the user had to paste by hand. Previously the hooks never fired until the user manually merged the snippet; now they go live on the next Claude Code session start.

Behavior change

This intentionally reverses the prior "ctx does not edit settings.json" contract. The old prose ("Merge this snippet into .claude/settings.json (ctx does not edit it)") is gone. The Edit(.claude/settings.json) deny rule still guards the file against later model edits.

Merge semantics (additive + idempotent, never clobbers)

• Missing file -> created from the snippet (Created).
• Existing JSON object -> deep-merged into a clone (Merged):
  • permissions.allow / permissions.deny: union with dedup by value; existing entries kept in order, snippet entries appended only if absent. Arrays/permissions created if missing.
  • hooks.SessionStart / hooks.PostToolUse / hooks.Stop: ctx's group appended to an event only if no existing group there references the substring .claude/hooks/ctx/ (the same idempotency marker the doctor uses). Events created if missing.
  • Every other key is left untouched.
  • If nothing changed, the file is not rewritten -> AlreadyWired (re-init leaves bytes identical).

Fallback safety

If .claude/settings.json exists but is not valid JSON (or not an object), it is left byte-for-byte untouched and init falls back to today's behavior: a stderr warning plus the snippet on stdout for manual merge (SkippedInvalid).

--json output gains a settings_action field (created / merged / already_wired / skipped_invalid); settings_snippet is retained.

Doctor's check_settings_wiring is unchanged (still valid for the manual-unwire / invalid-JSON cases).

Tests added

Unit (src/harness/mod.rs):

• test_wire_creates_settings_when_missing
• test_wire_merges_additively_preserving_user_settings
• test_wire_is_idempotent
• test_wire_leaves_invalid_json_untouched

CLI (tests/harness_cli.rs):

• test_init_merges_into_existing_settings_additively
• test_init_is_idempotent_on_settings
• test_init_leaves_invalid_settings_untouched_and_prints_snippet
• test_local_init_post_tool_use_hook_runs_end_to_end updated to assert settings.json is written and valid (old "snippet printed" assertions removed).

Verification

• cargo fmt, cargo test --verbose (all pass), cargo clippy -- -D warnings (clean).
• Manual smoke on a scratch dir: run 1 creates settings.json wiring the hooks; run 2 reports "already wired for ctx" with identical bytes; corrupting the file then re-running prints the warning + snippet and leaves the file byte-for-byte unchanged; ctx harness doctor reports no settings_not_wired.