feat(gitea): IssueTracker client + inject into brain server (#52)

Implements the IssueTracker port as a real Gitea REST client (#49c) — the
new outbound dependency the brain server gains for capture.

- CreateIssue / CommentIssue / CloseIssue(+optional closing comment) over
  the Gitea API. Owner is the const "mathias", never caller-supplied, so
  a caller cannot redirect a write to another owner's repo.
- Token read once at construction (BRAIN_GITEA_TOKEN), held in the struct,
  travels only in the Authorization header — never logged or in argv.
  Error messages carry status + truncated body, never the token
  (regression-tested). gitea.New returns nil when URL or token is unset,
  so missing config = tracker disabled via one nil check.
- Injected into the MCP server behind the capture.IssueTracker interface
  via WithIssueTracker (constructor injection, swappable/testable); main
  wires it from BRAIN_GITEA_URL (default https://git.d-ma.be) +
  BRAIN_GITEA_TOKEN. Consumed by the capture use-case in #53.

Tests use httptest transports: create (owner+auth header asserted),
comment, close with/without comment, error path that proves the token
never leaks into an error string.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/server_test.go

@@ -2,12 +2,14 @@ package mcp_test
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"net/http"
 	"net/http/httptest"
 	"strings"
 	"testing"
 
+	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/mcp"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -93,3 +95,22 @@ func TestServerUnknownMethodReturnsError(t *testing.T) {
 	assert.Equal(t, float64(-32601), errObj["code"])
 	assert.Contains(t, errObj["message"].(string), "unknown/method")
 }
+
+type stubTracker struct{}
+
+func (stubTracker) CreateIssue(context.Context, string, string, string) (capture.IssueRef, error) {
+	return capture.IssueRef{}, nil
+}
+func (stubTracker) CloseIssue(context.Context, string, int, string) (capture.IssueRef, error) {
+	return capture.IssueRef{}, nil
+}
+func (stubTracker) CommentIssue(context.Context, string, int, string) (capture.IssueRef, error) {
+	return capture.IssueRef{}, nil
+}
+
+func TestWithIssueTrackerInjects(t *testing.T) {
+	srv := mcp.NewServer(t.TempDir(), nil, nil, nil)
+	assert.Nil(t, srv.IssueTracker(), "tracker is off by default")
+	srv = srv.WithIssueTracker(stubTracker{})
+	assert.NotNil(t, srv.IssueTracker(), "tracker injected behind the interface")
+}