Add service layer and MCP server with read tools

Slice 1: internal/service is the one capability layer both the HTTP API and the MCP server call (AGENT.md 1.7); /api/repos and /api/repo route through it. Slice 2: internal/mcp serves an MCP server over Streamable HTTP at /mcp (go-sdk v1.8.0) with read tools list_repos and get_repo as thin adapters over the service, plus a round-trip test. Enabled air polling so hot reload works across the Docker-on-Windows bind mount.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/service/service.go

@@ -0,0 +1,47 @@
+// Package service is the ONE capability layer behind both the HTTP API and the
+// MCP server (AGENT.md §1.7). HTTP handlers and MCP tool handlers are thin
+// adapters that call these methods; Git/forge logic never lives in a handler.
+// Everything here goes through the internal/git (and later internal/forge)
+// boundaries and obeys the safety rules (§1.4).
+package service
+
+import (
+	"context"
+	"path/filepath"
+
+	"gitmanager/internal/git"
+	"gitmanager/internal/repos"
+)
+
+// Service holds the shared dependencies the capabilities need.
+type Service struct {
+	git   *git.CLI
+	index *repos.Index
+}
+
+// New builds a Service over the git boundary and the scanner's repo index.
+func New(g *git.CLI, index *repos.Index) *Service {
+	return &Service{git: g, index: index}
+}
+
+// ListRepos returns a snapshot of every discovered repository.
+func (s *Service) ListRepos() []repos.State {
+	return s.index.List()
+}
+
+// GetRepo returns the cached state for one repo, or false if it is not indexed.
+// The path is cleaned so separator style does not defeat the exact-match lookup.
+func (s *Service) GetRepo(path string) (repos.State, bool) {
+	return s.index.Get(filepath.Clean(path))
+}
+
+// RepoDetail returns the enriched detail (branches, commits, remotes) for one
+// repo. The second return is false when the path is not an indexed repository —
+// we never run git against an arbitrary caller-supplied path (§1.3).
+func (s *Service) RepoDetail(ctx context.Context, path string) (repos.Detail, bool) {
+	base, ok := s.index.Get(filepath.Clean(path))
+	if !ok {
+		return repos.Detail{}, false
+	}
+	return repos.BuildDetail(ctx, s.git, base), true
+}