feat(cli): add thread poke command

Re-runs the head step's agent with a supplementary prompt and replaces
the head step (rewires new step's prev to old head's prev) instead of
appending. Skips moderator re-route — the role of the head step is
reused.

Fixes #144

.changeset/thread-poke.md

@@ -0,0 +1,11 @@
+---
+"@united-workforce/cli": minor
+---
+
+feat(cli): add `uwf thread poke` command
+
+New subcommand `uwf thread poke <thread-id> -p <prompt>` re-runs the head step's
+agent with a supplementary prompt, replacing the head step's output. Unlike
+`thread resume`, poke skips the moderator and rewrites the new step's `prev`
+pointer so the new head replaces (not appends to) the old head. Works on idle
+and suspended threads. Resolves issue #144 (Phase 1).