Agent / Orchestrator: Handle merge conflicts in pr_iteration workflow - NEED DISCUSSION

### Component

None

### Describe the feature

When the agent creates a PR and the base branch evolves (someone pushes changes to main that touch the same files), the PR ends up with merge conflicts. Right now neither the agent nor the platform does anything about it - the PR just sits there as unmergeable and nobody notices until a human checks.

I hit this in practice: the agent opened a PR that created a `pyproject.toml` for a new project scaffold. Meanwhile I pushed a different `pyproject.toml` to main (production lib config, different name, version, dependencies). The PR is now in conflict and a `pr_iteration` to address review feedback would work on top of a stale branch without realizing the base has diverged. The agent would be editing a `pyproject.toml` that doesn't reflect what's actually on main.

### Use case

The agent created PR #5 with a `pyproject.toml` for project "xyz" v0.0.1. I then pushed to main a `pyproject.toml` for project "LEO" v1.0.1 with completely different config (production lib, different deps, different build system). The PR is now unmergeable but there's no mechanism to detect or resolve this. If I send a `pr_iteration` now, the agent will work on top of its own version of `pyproject.toml` without knowing main has a completely different one.

### How to reproduce

1. Submit a `new_task` that creates or modifies a file (e.g. "create a pyproject.toml for project xyz v0.0.1")
2. Wait for the agent to open the PR
3. While the PR is open, push a change to `main` that touches the same file (e.g. a different `pyproject.toml` with different project name, version, dependencies)
4. The PR is now unmergeable - GitHub shows "This branch has conflicts that must be resolved"
5. Submit a `pr_iteration` on that PR (e.g. to address review feedback)
6. The agent works on top of its own branch without realizing `main` has diverged - it edits a `pyproject.toml` that doesn't reflect what's actually on main

### Proposed solution

The `pr_iteration` workflow should check for conflicts before starting work:

1. **Detect** - at the start of `pr_iteration`, after checking out the PR branch, run `git merge origin/main --no-commit --no-ff` (or `git rebase origin/main --no-autostash`) to see if there are conflicts
2. **Resolve if possible** - if there are conflicts, the agent already has full context (it knows what the PR is doing and can read the incoming changes from main). Include in the `pr_iteration` prompt something like: "Before addressing review feedback, check if your branch has conflicts with the base branch. If it does, resolve them first - the base branch version takes priority unless the PR explicitly changes that file"
3. **Surface if not** - if the conflict is too complex (e.g. large structural changes), fail with a clear message: "PR has merge conflicts with main that need manual resolution" instead of silently working on a stale branch

This could also work as an automatic trigger: a webhook on the `pull_request` event with `action: "auto_merge_failed"` or a periodic check for unmergeable PRs that creates `pr_iteration` tasks to resolve them. But the simpler first step is just making `pr_iteration` conflict-aware.

### Other information

_No response_

### Acknowledgements

- [x] I may be able to implement this feature
- [ ] This might be a breaking change

ファイルやテストの名前は挙げられていません。まず pr_iteration ワークフローを見つけ、PR ブランチをチェックアウトする方法とベースブランチを特定する方法を確認してください。ここで説明されている競合した PR のシナリオを再現してください。完了条件は、反復作業の前に競合が検出され、可能な場合は解決され、または明確な失敗メッセージとして提示されることです。

索引モデルが issue の本文から書いたものです。