feat(hooks): guard subagent spawns (Task|Agent) toward graphify (#2145)

[ozdemirsarman]

Closes #2145.

Problem

The PreToolUse hook-guard registers only Bash|Grep and Read|Glob matchers, but
most exploration happens inside spawned subagents. The main model spawns an Explore
agent with "find all callers of X", and that agent never sees the guard — so the
graph-orientation nudge is bypassed exactly where it matters most.

Fix

Add a graphify hook-guard agent mode and register a Task|Agent PreToolUse matcher.
When a graph exists, the guard nudges the spawn (via additionalContext) to orient
through graphify before grepping/reading raw files — unless the subagent prompt is
already graphify-aware (mentions graphify, or carries pasted graph output like
Connections ( / [EXTRACTED]), in which case it stays quiet.

Advisory only: it never blocks a spawn, and fails open on an absent graph, an empty
brief, or malformed stdin — consistent with the existing search/read guards. The
install/uninstall filters are updated so the new matcher is registered and cleanly
removed alongside the others.

Tests

Adds tests/test_agent_hook.py (nudges on exploration spawns; silent without a graph,
on empty briefs, and when already graphify-aware; never blocks; valid PreToolUse JSON;
fails open on malformed stdin; uninstall filter strips ours but preserves a foreign
Task|Agent hook). Updates the two existing install tests that assert the guard count
(2 → 3). Full hook/install/settings suites pass.