Unify PR creation codepaths for conflict/non-conflict syncs. (#20)

## Description

Unifies the conflict and non-conflict codepaths in both
`_sync_private_to_public` and `_sync_public_to_private`.

Previously, a cherry-pick conflict caused the code to branch into a
completely separate `_handle_cherry_pick_conflict` function that
duplicated much of the standard flow (description building, trailer
appending, push, PR creation). This made it easy for the two paths to
drift — most recently, conflict PRs were getting `[CONFLICT]` prepended
to their titles and losing their original PR descriptions.

Now both functions follow a single linear flow with `is_conflict`
conditionals at the points where behavior differs:

1. Cherry-pick → set `is_conflict`
2. Commit handling (raw conflict commit + resolution agent vs. amend)
3. Build PR description (shared, identical logic)
4. Append trailers (origin always; conflict trailer only if conflict)
5. Push + create PR
6. Post-creation conflict steps if needed (label, reviewer, comment,
slack)

Other changes:
- Conflict PRs no longer get `[CONFLICT]` prepended to their title.
- Conflict PRs now preserve the original description (source PR body or
agent-generated) instead of replacing it with a generic conflict
message.
- A comment is posted on conflict PRs noting the reviewer assignment,
rather than baking that info into the PR body.
- Added `GhOps.add_pr_comment()` method.

## Testing

All 251 existing tests pass. Verified via manual inspection of a
conflict sync PR (warpdotdev/warp-internal#24195).

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: vorporeal

generate_jwt.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+
+client_id=$1 # Client ID as first argument
+
+pem=$( cat $2 ) # file path of the private key as second argument
+
+now=$(date +%s)
+iat=$((${now} - 60)) # Issues 60 seconds in the past
+exp=$((${now} + 600)) # Expires 10 minutes in the future
+
+b64enc() { openssl base64 | tr -d '=' | tr '/+' '_-' | tr -d '\n'; }
+
+header_json='{
+    "typ":"JWT",
+    "alg":"RS256"
+}'
+# Header encode
+header=$( echo -n "${header_json}" | b64enc )
+
+payload_json="{
+    \"iat\":${iat},
+    \"exp\":${exp},
+    \"iss\":\"${client_id}\"
+}"
+# Payload encode
+payload=$( echo -n "${payload_json}" | b64enc )
+
+# Signature
+header_payload="${header}"."${payload}"
+signature=$(
+    openssl dgst -sha256 -sign <(echo -n "${pem}") \
+    <(echo -n "${header_payload}") | b64enc
+)
+
+# Create JWT
+JWT="${header_payload}"."${signature}"
+printf '%s' "$JWT"

src/repo_sync/stack/gh_ops.py

@@ -247,6 +247,20 @@ def update_pr_body(self, pr_number: int, body: str) -> None:
         finally:
             os.unlink(body_file)
 
+    def add_pr_comment(self, pr_number: int, body: str) -> None:
+        """Add a comment to a PR."""
+        self._run(
+            [
+                "pr",
+                "comment",
+                str(pr_number),
+                "--repo",
+                self.repo,
+                "--body",
+                body,
+            ]
+        )
+
     def request_reviewer(
         self, pr_number: int, reviewer: str
     ) -> None: