feat(releaser): release v1.4.0 — GitHub, SSH agent, configurable bumps

- GitHub release support: internal/ghclient (no SDK, minimal HTTP client);
  GITHUB_TOKEN env var; github.token/github.repo config; GitHub takes
  precedence over GitLab when both are configured; publisher interface
  (releasePublisher) in cmd/main.go makes providers interchangeable
- SSH agent push: gitutil.Push() now tries gitssh.NewSSHAgentAuth for
  git@/ssh:// remotes before falling back to the system git binary
- --release-env-file flag: override dotenv artifact path; pass "" to disable
- git.releasable_types config: filter which commit types trigger a bump
  (defaults to fix, feat, breaking); useful for maintenance branches
- commits.Group(), ExtractSubject(), ReleasableSet() exported helpers:
  notes.go and changelog.go now delegate to these instead of duplicating
- CHANGELOG deduplication guard: changelog.Update() is idempotent — skips
  write if ## [version] section already exists
- Always load config sources: LoadWithSources() called unconditionally;
  removes the dual config path that previously only tracked sources in
  --verbose mode
- .releaser.gitlab-ci.yml: adds artifacts: reports: dotenv: release.env

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/ghclient/ghclient.go

@@ -0,0 +1,73 @@
+package ghclient
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+)
+
+// Client is a minimal GitHub API client covering only the Releases endpoint.
+type Client struct {
+	token      string
+	repo       string // "owner/repo"
+	httpClient *http.Client
+}
+
+// New creates a Client. repo must be in "owner/repo" format.
+func New(token, repo string) *Client {
+	return &Client{
+		token: token,
+		repo:  repo,
+		httpClient: &http.Client{
+			Timeout: 30 * time.Second,
+		},
+	}
+}
+
+type createReleaseRequest struct {
+	TagName string `json:"tag_name"`
+	Name    string `json:"name"`
+	Body    string `json:"body"`
+}
+
+// CreateRelease creates a GitHub release on an existing tag.
+// The tag must already be pushed to the remote before calling this.
+func (c *Client) CreateRelease(ctx context.Context, tagName, body string) error {
+	payload, _ := json.Marshal(createReleaseRequest{
+		TagName: tagName,
+		Name:    tagName,
+		Body:    body,
+	})
+
+	url := fmt.Sprintf("https://api.github.com/repos/%s/releases", c.repo)
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(payload))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+c.token)
+	req.Header.Set("Accept", "application/vnd.github+json")
+	req.Header.Set("X-GitHub-Api-Version", "2022-11-28")
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("GitHub API request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		var errBody struct {
+			Message string `json:"message"`
+		}
+		json.NewDecoder(resp.Body).Decode(&errBody) //nolint:errcheck
+		if errBody.Message != "" {
+			return fmt.Errorf("GitHub API returned %d: %s", resp.StatusCode, errBody.Message)
+		}
+		return fmt.Errorf("GitHub API returned %d", resp.StatusCode)
+	}
+
+	return nil
+}