Add agentpr pr edit subcommand

Update a PR's title and/or body as the agent user, sending only the
fields supplied.

AGENTS.md

@@ -8,9 +8,9 @@ from Vault, so actions are attributed to the agent rather than to whoever runs
 the tool. Setting `AGENT_LOGIN` selects a different agent identity, so a service
 like repospawner can run these tools as itself.
 
-- **`agentpr`** — create pull requests and post PR comments as `unkin-agent`
-  (fixes the "tea posts as Ben" attribution problem). Subcommands:
-  `pr create`, `pr comment`, `whoami`.
+- **`agentpr`** — create and edit pull requests, and post PR comments, as
+  `unkin-agent` (fixes the "tea posts as Ben" attribution problem).
+  Subcommands: `pr create`, `pr comment`, `pr edit`, `whoami`.
 - **`watchpr`** — poll one or more PRs and exit when a tracked PR changes
   meaningfully: it merges/closes, gets a new non-agent comment, its CI fails,
   or it loses mergeability. Benign transitions (CI pending→success, the agent's
@@ -28,7 +28,7 @@ parsing, watch-state comparison, git worktree helpers).
 ## Structure
 
 ```
-cmd/agentpr/main.go      # agentpr CLI (pr create / pr comment / whoami)
+cmd/agentpr/main.go      # agentpr CLI (pr create / pr comment / pr edit / whoami)
 cmd/watchpr/main.go      # watchpr CLI (poll + meaningful-change exit)
 cmd/agentws/main.go      # agentws CLI (new / list / rm / clean / token / credential)
 cmd/agentws/prune.go     # agentws prune (classify worktrees, remove the safe ones)
@@ -36,7 +36,7 @@ cmd/agentvault/main.go   # agentvault CLI (seed-outpost / seed-oauth)
 internal/agent/          # shared plumbing:
   token.go               #   env config + in-process Gitea-token cache
   vault.go               #   AppRole login + read the gitea creds path
-  gitea.go               #   Gitea REST client (PR create/get, comments, status, whoami)
+  gitea.go               #   Gitea REST client (PR create/edit/get, comments, status, whoami)
   parse.go               #   owner/repo#N and owner/repo parsing
   watch.go               #   PRState snapshot + MeaningfulChange comparison
   git.go                 #   git worktree/clone/fetch helpers (os/exec, no go-git)