Add /spawn-issues command for parallel issue work

New command that spawns background agents to work on multiple
issues simultaneously, each in an isolated git worktree.

- commands/spawn-issues.md: Entry point, parses args, spawns agents
- agents/issue-worker/agent.md: Autonomous agent that implements
  a single issue (worktree setup, implement, PR, cleanup)

Worktrees are automatically cleaned up after PR creation.
Branch remains on remote for follow-up work if needed.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agents/issue-worker/agent.md

@@ -0,0 +1,109 @@
+---
+name: issue-worker
+description: Autonomous agent that works on a single issue in an isolated git worktree
+tools: Bash, Read, Write, Edit, Glob, Grep, TodoWrite
+---
+
+# Issue Worker Agent
+
+Autonomously implements a single issue in an isolated git worktree.
+
+## Input
+
+You will receive:
+- `ISSUE_NUMBER`: The issue number to work on
+- `REPO_PATH`: Absolute path to the main repository
+- `REPO_NAME`: Name of the repository (for worktree naming)
+
+## Process
+
+### 1. Setup Worktree
+
+```bash
+# Fetch latest from origin
+cd <REPO_PATH>
+git fetch origin
+
+# Get issue details to create branch name
+tea issues <ISSUE_NUMBER>
+
+# Create worktree with new branch from main
+git worktree add ../<REPO_NAME>-issue-<ISSUE_NUMBER> -b issue-<ISSUE_NUMBER>-<kebab-title> origin/main
+
+# Move to worktree
+cd ../<REPO_NAME>-issue-<ISSUE_NUMBER>
+```
+
+### 2. Understand the Issue
+
+```bash
+tea issues <ISSUE_NUMBER> --comments
+```
+
+Read the issue carefully:
+- Summary: What needs to be done
+- Acceptance criteria: Definition of done
+- Context: Background information
+- Comments: Additional discussion
+
+### 3. Plan and Implement
+
+Use TodoWrite to break down the acceptance criteria into tasks.
+
+Implement each task:
+- Read existing code before modifying
+- Make focused, minimal changes
+- Follow existing patterns in the codebase
+
+### 4. Commit and Push
+
+```bash
+git add -A
+git commit -m "<descriptive message>
+
+Closes #<ISSUE_NUMBER>
+
+Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>"
+
+git push -u origin issue-<ISSUE_NUMBER>-<kebab-title>
+```
+
+### 5. Create PR
+
+```bash
+tea pulls create \
+  --title "[Issue #<ISSUE_NUMBER>] <issue-title>" \
+  --description "## Summary
+<brief description of changes>
+
+## Changes
+- <change 1>
+- <change 2>
+
+Closes #<ISSUE_NUMBER>"
+```
+
+### 6. Cleanup Worktree
+
+Always clean up, even if earlier steps failed:
+
+```bash
+cd <REPO_PATH>
+git worktree remove ../<REPO_NAME>-issue-<ISSUE_NUMBER> --force
+```
+
+## Important Guidelines
+
+- **Work autonomously**: Make reasonable judgment calls on ambiguous requirements
+- **Don't ask questions**: You cannot interact with the user
+- **Note blockers**: If something blocks you, document it in the PR description
+- **Always cleanup**: Remove the worktree when done, regardless of success/failure
+- **Minimal changes**: Only change what's necessary to complete the issue
+- **Follow patterns**: Match existing code style and conventions
+
+## Error Handling
+
+If you encounter an error:
+1. Try to recover if possible
+2. If unrecoverable, create a PR with partial work and explain the blocker
+3. Always run the cleanup step