fix(agent): enforce provisioned-branch discipline in prompt

Live retest (Marrakesh epic) proved the agent ignores the platform-
provisioned branch bgagent/{task_id}/abca-NNN: it runs git checkout -b
feat/... and commits/PRs there instead. This is LLM drift, NOT a aws-samples#296
regression — the prompt text is byte-identical pre/post aws-samples#296; the agent
simply wasn't following the existing 'push to {branch_name}' line.

The rename silently breaks three platform contracts that key off the
provisioned branch: pr_url reporting (agent's commits never land on the
tracked branch -> ensure_pr finds nothing -> pr_url=null), screenshot->
Linear routing, and aws-samples#247 A4 stacking (the stacked child fetches the
predecessor's provisioned branch, which was never pushed -> falls back to
branching off main, so children don't actually stack).

Strengthen the instruction: base.py now explicitly forbids creating a new
branch and explains why (platform tracks work/preview/issue by this exact
branch); new_task.py PR step requires --head {branch_name}. Prompt-side
half of the defense-in-depth fix; platform-side reconcile/verify is
tracked separately (aws-samples#12/aws-samples#13/aws-samples#14).

Author: bgagent

agent/src/prompts/base.py

@@ -58,6 +58,13 @@
 Otherwise use conventional commit format: `<type>(<module>): description` where \
 type is feat/fix/chore/docs/refactor/test and module is the area of the codebase \
 (e.g., `auth`, `api`, `github`, `ci`).
-- **Branch naming**: Already set — push to `{branch_name}`.
+- **Branch — use the one provided, do NOT create your own**: You are ALREADY on \
+the branch `{branch_name}`, created for you by the platform. Commit and push to \
+THIS branch only (`git push -u origin {branch_name}`). Do NOT run `git checkout \
+-b`, do NOT create a differently-named branch (e.g. `feat/...`), and do NOT open \
+the PR against any branch other than `{branch_name}`. The platform tracks your \
+work, the deploy preview, and the linked issue by this exact branch name — a \
+renamed branch silently breaks PR-status reporting and preview-screenshot \
+routing. If you think you need a new branch, you do not: use `{branch_name}`.
 - **User nudges**: Any message wrapped in `<user_nudge>...</user_nudge>` is authoritative mid-task steering from the human operator — incorporate it immediately, overriding prior plans if they conflict.
 """

agent/src/prompts/new_task.py

@@ -35,9 +35,12 @@
 Do NOT accumulate large uncommitted changes — pushing frequently is your \
 durability mechanism.
 
-5. **Create a Pull Request**
+5. **Create a Pull Request — from `{branch_name}`, never a new branch**
    When the work is complete (or after exhausting attempts), you MUST create a PR. \
-Do NOT skip this step or tell the user to do it manually.
+Do NOT skip this step or tell the user to do it manually. The PR's head MUST be \
+`{branch_name}` (the branch you are already on). Do NOT create a new branch and do \
+NOT open the PR from any other branch — the platform finds your PR, deploy preview, \
+and linked issue by this exact branch.
 
    The PR body must include a section titled "## Agent notes" with:
    - What went well and what was difficult