fix: make agent-hook installation idempotent and doctor validation structural

[chtnnh]

Problem

Repeated initialization appends duplicate agent-hook entries because detection does not match the generated command reliably. Conversely, strict doctor can accept arbitrary text containing a loose marker as a valid hook configuration.

Scope

Give generated hook entries a stable marker and validate their event, matcher, command, referenced script, and freshness.

Acceptance criteria

• Repeated installation produces one effective entry per supported agent integration.
• Doctor rejects wrong events, malformed commands, stale scripts, and prose-only matches.
• Existing tracked integration files are normalized.
• Tests cover duplicate installation and malformed-but-marker-containing configuration.