[BUG] isolation: "worktree" does not create git worktrees for plugin-defined agent types

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Description

When spawning agents via the Agent tool with isolation: "worktree" and a plugin-defined subagent_type (e.g., "my-plugin:Software-Engineer"), no git worktrees are created. The parameter is accepted without error, but agents all share the main working directory.

Expected: Each agent gets its own worktree in .claude/worktrees/ with a separate branch, as documented in the Agent tool description.

Actual: git worktree list shows only the main worktree. All agents operate in the same directory and overwrite each other's branches.

## Evidence

PreToolUse hook logging confirms the parameter is present on the Agent tool call:

SUBAGENT_TYPE=my-plugin:Software-Engineer
TEAM_NAME=my-sprint
HAS_ISOLATION=yes ← parameter IS set

PostToolUse hook logging confirms no worktrees were created:

WORKTREE_COUNT=1 ← only main worktree exists
LIST=/path/to/project abc1234 [main]

Additionally, PostToolUse:Agent hook error is shown in the UI after every Agent spawn with plugin types, even when the PostToolUse hook script produces valid JSON output (247 bytes, verified via debug logging).

## Environment

• Claude Code CLI (latest)
• macOS (also expected on Linux/Windows)
• Plugin installed via git marketplace (extraKnownMarketplaces in settings.json)

## Workaround

Manually create worktrees before spawning agents:

```bash
git worktree add .claude/worktrees/feature-a -b feature-a-branch
git worktree add .claude/worktrees/feature-b -b feature-b-branch

Then instruct agents to work in their assigned worktree directory.

Additional Notes

• The same PostToolUse:Agent hook error appears even for hooks that produce valid JSON with additionalContext — suggesting PostToolUse hooks may not be fully supported for the Agent tool.
• Built-in subagent_type values (e.g., "general-purpose", "Explore") were not tested with isolation: "worktree" in this scenario — the issue may be specific to plugin-defined types.

What Should Happen?

Each agent gets its own worktree in .claude/worktrees/ with a separate branch, as documented in the Agent tool description.

Error Messages/Logs

Steps to Reproduce

1. Install a Claude Code plugin that defines custom agent types (via plugin.json agents field)
2. Create a team with TeamCreate
3. Spawn multiple agents using the plugin agent type with isolation: "worktree":

Agent(
subagent_type: "my-plugin:Software-Engineer",
team_name: "my-team",
isolation: "worktree",
prompt: "..."
)

1. Run git worktree list — only the main worktree appears
2. All spawned agents share the same working directory

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.81 (Claude Code)

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_