feat: cross-runner handoff (/handoff claude|codex) for Linear issues

[hcaumo]

Summary

Adds cross-runner handoff to Cyrus: a Linear issue started with one runner (Claude or Codex) can be handed to the other so it continues from the existing work, files, branch, and PR state — in the same worktree, never running both runners at once.

@Cyrus /handoff codex     # hand the work to Codex
@Cyrus /handoff claude    # hand the work to Claude

Initial-runner tags ([agent=claude] / [agent=codex]) are unchanged.

How it works

• HandoffService (packages/edge-worker/src/HandoffService.ts) — command parsing (parseHandoffCommand), worktree/git/PR snapshot (buildSnapshot), target-prompt formatting (buildHandoffPrompt), a stop-poller (waitForStopped), and getActiveRunnerType.
• runnerTypeOverride threaded through RunnerConfigBuilder.buildIssueConfig → EdgeWorker.buildAgentRunnerConfig → resumeAgentSession. When set, it forces the target runner, bypassing label/description selection and the sticky-session binding. Defaults to undefined, so existing single-runner behavior is byte-for-byte unchanged.
• Follow-up path (handleHandoffCommand): stops the active runner (sequentially, with a ~30s timeout → "handoff blocked" comment), snapshots the worktree state, clears the old runner binding, and starts the target in the same session.workspace.
• Creation path: Linear @mentions create a new agent session, so @Cyrus /handoff codex arrives on the session-creation webhook. The creating comment is parsed and the requested runner is forced for the new session (the issue's worktree is reused, so the target picks up prior files/commits/PR).
• Best-effort git/PR snapshot reads added to GitService; getLastAssistantBody / clearRunnerSessionBindings added to AgentSessionManager.

Testing

• New unit/integration tests: command parsing, runner-type detection, snapshot building, prompt formatting, stop-poller, runnerTypeOverride, same-worktree reuse, blocked handoff, failure handling, and creation-path handoff selection. Full cyrus-edge-worker suite: 765 passing; pnpm typecheck clean.
• Validated live against Linear on a self-hosted Cyrus: @Cyrus /handoff codex on a Claude-started issue stopped Claude and started a Codex runner in the same worktree, continuing the work.

Acceptance criteria

• [agent=claude] / [agent=codex] still start exactly one runner (regression-guarded).
• /handoff codex after Claude starts Codex in the same worktree; /handoff claude after Codex starts Claude.
• Never runs both runners concurrently in the same worktree.
• Both runners report to the same Linear issue.
• Tests cover parsing, runner selection, same-worktree reuse, blocked handoff, snapshot creation, and failure handling.

Changelog updated under ## [Unreleased].