fix(hooks): route rtk hook check through the real decision

`rtk hook check` called `registry::rewrite_command` directly, with no
permission verdict and none of the gates the hooks apply. It therefore
reported a rewrite for command substitutions, file redirects and heredocs
that both hook paths refuse to touch -- the diagnostic disagreed with the
thing it exists to diagnose, and did so in the direction that matters, by
claiming RTK would rewrite a command it deliberately leaves alone.

Route it through `hooks::decision` so it answers the same question, and
report a deny rule distinctly from "no rewrite" rather than collapsing both
into one message. Both still exit 1.

That makes the answer agent-dependent, so `--agent` stops being discarded.
`AgentPath` records what actually differs between agents, which is whose
permission rules their hook consults: the six that decide in-process via
`rtk hook <agent>` use their own host's rules, the five whose plugin shells
out to `rtk rewrite` get Claude's (that entry point cannot be told who is
asking), and the six that install only a rules file have no hook and so no
rules at all. Every install target resolves -- including `codex` and
`openclaw`, which are install flags rather than `AgentTarget` variants --
and only a genuine typo is rejected.

What does *not* differ is a rewrite that changed nothing: every agent
discards it, the in-process hosts in `hook_cmd` and the others in their own
plugin, since `hooks/opencode/rtk.ts`, `hooks/pi/rtk.ts` (shared with omp)
and hermes' `__init__.py` all gate on `rewritten != command`. `AgentPath`
suppresses it for every variant. Only the bare `rtk rewrite` CLI reports the
no-op, and no agent consumes that answer raw.

Consulting no rules and ignoring `--agent` is what made the diagnostic
contradict every host: under a Claude deny rule for `git status` it reported
`rtk git status` while `rtk hook claude` refused the command outright; it
reported a rewrite for `rtk git status`, which no agent applies; and since
`--agent` selected nothing, the answer described no host in particular.

The expectations pinned in the characterization commit are updated here, in
the same commit, so the behavior change is visible as a diff rather than as a
test that quietly stopped asserting. That harness now also asserts the hook
exits 0 and never panics: a crash produces empty stdout, which would let every
"expect no output" assertion pass vacuously. The agent list is derived from
`AgentTarget::value_variants()` so a new variant fails the test instead of
silently becoming unanswerable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>