docs(file_write_branch): clarify direct-to-main + PR-flow in descriptor (#35)

The tool already commits directly to any existing branch (BranchExists→upsert,
no create — covered by TestFileWriteBranchSkipsCreateWhenBranchExists), so
`branch:"main"` is a one-call direct-to-main write. The descriptor said "feature
branch", understating it. pr_create/pr_merge/repo_list already exist, closing the
other two #35 gaps (PR loop + owner repo listing). Descriptor now states both
paths.

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

internal/tools/file_write_branch.go

@@ -23,7 +23,7 @@ func NewFileWriteBranch(c *gitea.Client, a *allowlist.Allowlist) *FileWriteBranc
 func (t *FileWriteBranch) Descriptor() registry.ToolDescriptor {
 	return registry.ToolDescriptor{
 		Name:        "file_write_branch",
-		Description: "Create or update a file on a feature branch. Branch is created from base if it doesn't exist.",
+		Description: "Create or update a file on the given branch. Pass an existing branch — e.g. the default branch `main` — to commit directly to it (trunk-based); a branch that doesn't exist yet is created from `base` first (PR flow). Pair with pr_create/pr_merge for the branch→PR→merge path.",
 		InputSchema: json.RawMessage(`{
 			"type":"object",
 			"properties":{