setup: write the agent-guard hook to gitignored settings.local.json instead of committing it to settings.json

[milamberspace]

Problem

Today the adopt flow commits the agent-guard PreToolUse hook wiring into the
tracked .claude/settings.json, while the script it invokes
(.claude/hooks/agent-guard.py) stays gitignored and is only synced in by
/magpie-setup. That split has caused a chain of follow-up fixes:

• fix(agent-guard): make the PreToolUse hook a no-op until agent-guard.py is present #786 — a fresh clone has the committed wiring but not yet the script, so
  the hook command had to be guarded ([ -f … ] && python3 … || true) to
  no-op when the script is absent.
• fix(agent-guard): make the fresh-clone hook guard cross-platform (Windows) #823 — that guard is a POSIX-shell conditional, which breaks on Windows
  (PowerShell, no Git Bash), so it had to be rewritten as an inline
  python3 -c existence check.

Both are workarounds for the same root cause: we commit machine-specific hook
wiring for a script we deliberately don't commit. It also enlarges the
always-committed footprint for an opt-in feature.

Proposal

Have /magpie-setup write the hook entry into the gitignored, per-machine
.claude/settings.local.json at the same moment it deposits
agent-guard.py — and stop committing it in .claude/settings.json.

Because the wiring then only ever exists once the script is present:

• No fresh-clone guard needed (fix(agent-guard): make the PreToolUse hook a no-op until agent-guard.py is present #786 becomes moot) — the command can be a
  plain python3 "$CLAUDE_PROJECT_DIR/.claude/hooks/agent-guard.py".
• No cross-platform shell gymnastics needed (fix(agent-guard): make the fresh-clone hook guard cross-platform (Windows) #823 becomes moot) — plain
  python3 "…" runs on POSIX and Windows alike.
• Zero committed footprint for the hook — nothing tracked in the adopter
  repo, matching the fact that both the script and its trigger are
  per-machine framework artefacts, not project config.

.claude/settings.local.json is Claude Code's documented per-user override
file and is already the natural home for machine-local settings; adopters
gitignore it (as this repo's .gitignore already does for settings.local).

Scope of the framework change

• skills/setup/adopt.md — Step 12 (and the verify step): write/merge the
  hook block into .claude/settings.local.json instead of surfacing a
  settings.json snippet for the maintainer to commit; drop the -f/guard.
• .gitignore template — ensure /.claude/settings.local.json is ignored.
• tools/agent-guard/README.md — document the settings.local.json wiring
  and the plain command.
• unadopt/verify flows — read/clean settings.local.json.
• Idempotent merge: don't clobber an adopter's existing
  settings.local.json keys.

Context

Raised from Apache JMeter's adoption review. The JMeter PMC reviewer (@vlsi)
independently asked "Can magpie add the hook entry to
.claude/settings.local.json on setup?" — this issue is that request,
generalised. JMeter has already dropped the committed .claude/settings.json
from its adoption PR in anticipation.

If accepted, this supersedes #823 (and retro-justifies removing the #786
guard).

[onlyarnav]

Is this issue still relevant and need work?
I can't seem to understand if this requires contribution?
Please lmk, Thanks!