feat(bootstrap): allow lifecycle scripts only for agent-browser in SecStack profile

[eggmasonvalue]

Summary

Modern npm (npm 11.17+ and npm 12 under RFC 868) blocks lifecycle scripts for installed dependencies by default. agent-browser relies on "postinstall": "node scripts/postinstall.js" to download/verify platform binaries, ensure executable permissions (chmod 0o755), write the .install-method marker, and configure native shims. Without script permission, npm silently skips the postinstall hook.

Because SecStack installs npm packages using --prefix ~/.pi/secstack-agent/npm (project-scoped), passing CLI flags (--allow-scripts) or environment variables (npm_config_allow_scripts) throws EALLOWSCRIPTS. Furthermore, configuring user-level ~/.npmrc pollutes global npm configuration and triggers warnings across non-Pi projects.

This PR configures allowScripts in the SecStack profile's npm root manifest (~/.pi/secstack-agent/npm/package.json):

• Bootstrap flow: scripts/bootstrap.mjs ensures ~/.pi/secstack-agent/npm/package.json contains "allowScripts": { "agent-browser": true } before running pi install npm:agent-browser.
• secpi update --extensions flow: When Pi updates npm packages within the profile, npm reads the manifest, permits the postinstall script for newer versions of agent-browser, and natively preserves the allowScripts field when writing back package.json.
• Isolation: Lifecycle scripts remain permitted strictly for agent-browser within the profile's npm root, leaving all other packages blocked and user projects unaffected.

Verification

1. Verified node --check scripts/bootstrap.mjs passes.
2. Verified ensureNpmAllowScripts() idempotently merges "allowScripts": { "agent-browser": true } into ~/.pi/secstack-agent/npm/package.json.
3. Verified npm rebuild agent-browser --foreground-scripts and PI_CODING_AGENT_DIR="$HOME/.pi/secstack-agent" pi update --extensions cleanly execute postinstall.js and mark the native binary executable without errors or warnings.