Config: .env = projects root only; derive each project's forge from git

Reduce .env to just REPOS_HOST_PATH (the projects root); runtime bootstrap moves to compose/defaults. Projects are the subdirectories of the single root — removed the project-directories feature (store table, service methods, /api/config/project-dirs, Settings section). Each project's forge is derived from its git remote (matched to a configured forge, else the bare host) and shown as a pill next to its name (State.Forge via scanner ForgeFor + svc.ForgeDisplay). Removed first-run .env seeding; forges + identity are managed in Settings. Added forge.HostOf. AGENT.md updated.

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

internal/forge/forge.go

@@ -66,6 +66,16 @@ type Provider interface {
 	MergeAndCleanup(ctx context.Context, owner, repo string, number int64, method MergeMethod) (MergeResult, error)
 }
 
+// HostOf returns the hostname of a base URL (e.g. "https://git.example.com:3000"
+// → "git.example.com"), or "" if it can't be parsed.
+func HostOf(rawURL string) string {
+	u, err := url.Parse(strings.TrimSpace(rawURL))
+	if err != nil {
+		return ""
+	}
+	return u.Hostname()
+}
+
 // ParseRemote extracts (host, owner, repo) from a git remote URL, handling both
 // https ("https://host/owner/repo.git") and scp-like ssh ("git@host:owner/repo.git").
 func ParseRemote(remote string) (host, owner, repo string, ok bool) {