feat: rewrite pass_mgr with build-time AES key, update pcexec & install

pass_mgr:
- Complete rewrite using build-time AES key (injected via ldflags)
- New command format: get-secret/get-username --key, set --key --secret
- Admin commands: init, handoff, init-from (rejected when AGENT_* env set)
- Inline pcguard check for agent commands
- Legacy 'get <key>' kept for backward compat
- Storage: pc-pass-store/<agent-id>/<key>.gpg with AES-256-GCM
- Admin password stored as SHA-256 hash in .pass_mgr/admin.json

pcexec.ts:
- Support new 'get-secret --key' pattern alongside legacy 'get <key>'
- Pass environment to fetchPassword for pcguard validation
- Deduplicate matches, sanitize all resolved passwords from output

install.mjs:
- Generate random 32-byte hex build secret (.build-secret)
- Reuse existing secret on rebuilds
- Pass to go build via -ldflags -X main.buildSecret=<secret>

README.md:
- Document new pass_mgr command format
- Document admin handoff/init-from workflow
- Document security model limitations
- Update project structure

.gitignore

@@ -31,3 +31,6 @@ vendor/
 
 # Lock files (sub-packages)
 plugin/package-lock.json
+
+# Build secret (generated by install.mjs)
+.build-secret