fix(skills): route show/info/list-filter to local, not model invoke

[code-yeongyu]

Problem

claw skills show plan, claw skills describe plan, claw skills list plan, claw skills info plan were falling through to SkillSlashDispatch::Invoke, spawning real model sessions and consuming tokens.

claw skills show plan --output-format json     # → model session, $0.1381
claw skills help plan --output-format json     # → model session, $0.1381
claw skills describe plan --output-format json # → model session, $0.0280
claw skills info plan --output-format json     # → hung/killed
# session count 43 → 50 across four top-level session files

Root cause: classify_skills_slash_command had no guards for discovery prefixes — every non-reserved arg became Invoke("$<arg>").

Fix

• Add "show" | "info" | "describe" as Local-only bare tokens
• Add starts_with guards for "show ", "info ", "list ", "describe " → Local
• handle_skills_slash_command + _json: filter skill list by name for show/info/describe, by substring for list — no model call, no session created

Test

skills_show_and_list_filter_do_not_invoke_model asserts all discovery patterns return Local, and bare skill names still dispatch to Invoke.

Pinpoints: ROADMAP #502, #503

[code-yeongyu]

QA note: resumed_status_command_emits_structured_json_when_requested fails on this branch but this is a pre-existing failure on main — not introduced by this PR. The test expects exit 0 but gets exit 1 for --resume SESSION /status --output-format json. This is a main regression that needs fixing separately before these PRs can show clean CI on that specific test.