feat: implement claude-sync auto enable/disable/status

[jimjawn]

Summary

Implements the claude-sync auto subcommand that was documented in the README but not yet functional.

claude-sync auto enable   # Install auto-sync hooks into Claude Code
claude-sync auto disable  # Remove auto-sync hooks
claude-sync auto status   # Show current hook status

How it works

Reads and writes ~/.claude/settings.json using the Claude Code hook format. On enable, two hooks are added:

• SessionStart → claude-sync pull -q (pull latest on session start)
• Stop → claude-sync push -q (push changes on session end)

On disable, only the claude-sync entries are removed — all other hooks are left exactly as they were.

Implementation details

• New package internal/claudesettings handles settings.json read/write
• Uses a raw map[string]json.RawMessage round-trip so all unknown fields are preserved — no data loss on save
• Settings file written with 0600 permissions
• Idempotent: enable is a no-op if hooks already present, disable is a no-op if not installed
• Creates ~/.claude/settings.json if it doesn't exist yet

Tested

• status correctly reports not-installed / enabled state
• enable merges with existing hooks (pre-existing Stop hooks survive intact)
• disable removes only claude-sync entries, leaves other hooks untouched
• Round-trip verified: settings.json fields outside hooks are preserved

Closes the gap noted in issue #26.