chore: deny unreviewed dependency install scripts

[clappingmonkey]

What & why

npm 11 warns on every npm ci that msgpackr-extract has an install script nobody has reviewed. It comes in via @opencode-ai/plugin → effect → msgpackr.

• package.json: adds allowScripts: { "msgpackr-extract": false }. It's an optional native speedup: msgpackr falls back to pure JS, and we only import types from the plugin package.
• .npmrc: adds strict-allow-scripts=true, so a new unreviewed install script fails npm ci instead of only warning. npm 12 will block these by default.

Consumers are unaffected. allowScripts is only read from the root project, and .npmrc is never packed. Renovate runs npm with scripts disabled, so it can still open PRs. CI on a PR that adds a new install-script dependency will fail until the script is approved or denied with npm install-scripts approve|deny <pkg>.

Closes: N/A (audit item C7)

Behavior impact

• Theme-switching logic (periodFor, boundaries, gentle override)
• Plugin options (dayTheme, nightTheme, dayStartHour, nightStartHour,
  checkIntervalMs, toast) — README options table updated
• Lifecycle / timer / toast behavior
• Packaging (package.json exports, files, published contents)
• No user-facing behavior change

Testing

• npm run typecheck passes
• npm test passes (22/22)
• Manually verified in opencode (if behavior changed): not applicable, no behavior change

Also verified:

• npm ci no longer prints the install-scripts warning. With the deny entry removed, it fails with --strict-allow-scripts, as intended.
• npm pack --dry-run lists only LICENSE, README.md, package.json and src/index.ts.

Checklist

• PR title follows Conventional Commits
  (enforced by the PR Title check; drives the release-please version bump)
• Defaults still work with zero config
• Docs updated if behavior/config changed (no behavior/config change)
• No secrets or credentials introduced