chore: update gitignore, checksums, and docs

- Exclude AI agent instruction files from git tracking
  (AGENTS.md, CLAUDE.md, GEMINI.md, .github/copilot-instructions.md)
- Add .geminiignore
- Regenerate lib/checksum.txt to reflect updated scripts
- Document sync-init.sh usage in docs/README.md
- Remove deprecated git.io short URLs from README

Author: ss-o

.geminiignore

@@ -31,13 +31,40 @@
   - https://wiki.zshell.dev
 - Loader:
   - https://init.zshell.dev
-  - https://git.io/zi-loader
 - Installer:
   - https://get.zshell.dev
-  - https://git.io/get-zi
 - R2:
   - https://r2.zshell.dev
 - IPFS:
   - https://ipfs.zshell.dev
 - jsDeliver:
   - https://cdn.jsdelivr.net/gh/z-shell/zi-src@main/
+
+### Maintainer — Verify and Sync Loader
+
+Check whether the local `lib/zsh/init.zsh` matches the canonical GitHub raw `main` copy:
+
+```sh
+sh lib/sh/sync-init.sh
+```
+
+Replace the local file if it drifts:
+
+```sh
+sh lib/sh/sync-init.sh --write
+```
+
+Run against local fixtures (no network required, useful in tests):
+
+```sh
+sh lib/sh/sync-init.sh \
+  --local  /tmp/my-init.zsh \
+  --remote /tmp/remote-init.zsh \
+  --checksum-url /tmp/checksum.txt
+```
+
+Skip checksum validation:
+
+```sh
+sh lib/sh/sync-init.sh --no-checksum
+```

docs/README.md

@@ -1,3 +1,4 @@
-13668e0d6edce92994b0434932873d8a8a2c2579a2cb53964de19212f02a2954 lib/sh/install_zpmod.sh
-6214cad3026f4150dfe1070592f6b24256cae5c60c596bdfebda07ffeaaf39fc lib/sh/install.sh
-23a563e80249a866c7cba3ac44eaedb1ca38f20c35690fb764a7cb75e95d38be lib/zsh/init.zsh
+5d8fdd881b84548f38c1d5ffc6d5f46a18919d8dd6c1398d69abe055dd76ef98  lib/sh/install_zpmod.sh
+9d643c40be25f71d93dd295dacc58944defe7218f961f3f3ecb4ee9c0a56d486  lib/sh/install.sh
+83d9f0dc013376b5aa03b7a11f3908058da176df3407ac6e2872857d7c9658fd  lib/sh/sync-init.sh
+66dc5bb0575d44e4e1192e229204abe1ac47979a05ee3c3bb23afc2b9b493637  lib/zsh/init.zsh