feat(#52): add agent_skills_security.trusted_global_roots allowlist for global skills (#754)

* feat(#52): add agent_skills_security.trusted_global_roots allowlist

Opt-in allowlist so a global: agent skill whose SKILL.md realpath resolves
outside the default global skills base (e.g. ~/.claude/skills) is accepted
when its real target lies under a user-declared trusted root. Default [] is
byte-identical to prior behavior; the symlink-escape guard is preserved and
simply re-applied against each declared root.

- src/security.cts: loadTrustedGlobalRoots — tilde-expand (~ and ~/), reject
  project-relative and dangerously broad roots (filesystem/UNC root, homedir),
  realpath-canonicalize each root every run and drop non-existent ones.
- src/init.cts: on base-check failure the guard consults the trusted roots
  (hoisted out of the loop); emits a stderr NOTE when a skill is accepted via
  a trusted root so the widened boundary is visible.
- src/core.cts: thread agent_skills_security through loadConfig.
- config-schema.manifest.json: allow the new key path.
- docs/CONFIGURATION.md: document the option and its security model.
- tests/agent-skills.test.cjs: unit + end-to-end CLI coverage (regression,
  feature, negative, broad-root hardening, stderr NOTE).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs(#52): add changeset fragment for trusted_global_roots (#754)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/core.cts

@@ -536,6 +536,7 @@ function loadConfig(cwd: string, options: Record<string, unknown> = {}): Record<
       effort: (parsed['effort']) || null,
       fast_mode: (parsed['fast_mode']) || null,
       agent_skills: (parsed['agent_skills']) || {},
+      agent_skills_security: (parsed['agent_skills_security']) || null,
       manager: (parsed['manager']) || {},
       response_language: get('response_language') || null,
       claude_md_path: get('claude_md_path') || null,