fix(fp-check): scope hooks and make Stop path-aware

[GotenJBZ]

Fixes #143. Supersedes #188.

Problems (in main)

1. Stop and SubagentStop both use matcher: "*", so they run on every subagent/agent stop in any session where fp-check is installed.
2. The Stop prompt demands the full deep-path output (5 phases, 13 questions, full PoC) for every bug. Standard-path verifications are rejected as "incomplete".
3. Hook loops in the main agent when running fp-check using subagents/ultracode.

Changes

1. Stop hook moved into skill frontmatter. This way, the stop hooks will be executed only when the skill is loaded, avoiding the firing in unrelated sessions.
2. Changed the Stop hook prompt as ordered rules: (1) skip if not fp-check (2) skip if delegated to subagents/workflow, (3) check only the path actually used (standard vs deep)
3. SubagentStop remains in hooks.json, narrowing the matcher to only support specific subagents data-flow-analyzer|exploitability-verifier|poc-builder

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[GotenJBZ]

Adding as a reviewer both @remiforall and @computerality since they both tried to solve this exact problem.

EDIT: I cannot do that :ops: