feat: 新增 Git credential helper 协议支持，实现 get|store|erase 子命令

README.md

@@ -160,6 +160,63 @@ quicommit profile stats
 quicommit profile token
 ```
 
+### Git Credential Helper
+
+QuiCommit can act as a Git credential helper to securely store and retrieve
+Personal Access Tokens (PATs) via the system keyring. The `credential` command
+is hidden from `--help` because it is invoked automatically by Git, not by end
+users.
+
+#### Setup
+
+Register QuiCommit as a credential helper for Git:
+
+```bash
+# Use the default QuiCommit config
+git config --global credential.helper quicommit
+
+# Or specify a custom config file
+git config --global credential.helper "quicommit --config /path/to/config.toml"
+```
+
+You can also limit the helper to a specific host:
+
+```bash
+git config --global credential.https://github.com.helper quicommit
+```
+
+#### How It Works
+
+When Git needs credentials (e.g. pushing to a remote), it calls the helper
+following the [gitcredentials protocol](https://git-scm.com/docs/gitcredentials):
+
+1. **`get`** — Git asks QuiCommit for a stored PAT matching the requested host.
+   QuiCommit searches all configured profiles in the keyring and returns the
+   PAT (plus username) if found.
+2. **`store`** — After a successful authentication (e.g. you entered a PAT in
+   the Git prompt), Git asks QuiCommit to save it. The PAT is stored in the
+   system keyring, bound to the matching profile.
+3. **`erase`** — Git asks QuiCommit to delete a stored PAT for the given host.
+
+Host names are mapped to canonical service names (`github.com` → `github`,
+`gitlab.com` → `gitlab`, `bitbucket.org` → `bitbucket`, etc.). Unknown hosts
+are used as-is.
+
+#### Removing Stored Credentials
+
+To remove a stored PAT, you can either use Git's built-in mechanism:
+
+```bash
+echo "protocol=https
+host=github.com" | git credential reject
+```
+
+Or remove it via the profile token management command:
+
+```bash
+quicommit profile token
+```
+
 ### Configure LLM
 
 ```bash
@@ -239,6 +296,7 @@ quicommit config reset --force
 | `quicommit changelog` | `cl` | Generate changelog |
 | `quicommit profile` | `p` | Manage Git profiles |
 | `quicommit config` | `cfg` | Manage settings |
+| `quicommit credential` | — | Git credential helper (hidden, invoked by Git) |
 
 ### Commit Options