feat(plan-mode): enhanced plan mode with explicit step tracking (#694)

Changes from the original:
- Explicit [DONE:n] tag tracking (more accurate than auto-marking on tool_result)
- Plan: header requirement - only extracts todos from 'Plan:' sections
- Utils extracted to separate file for testability
- Better session resume - only scans messages after plan-mode-execute marker
- Context filtering - properly filters plan-mode-context custom type messages
- Refactored to directory structure (index.ts + utils.ts + README.md)

The original auto-completed steps on every tool_result, which was inaccurate
for multi-tool steps. This version uses explicit [DONE:n] markers that the
agent outputs after completing each step.

packages/coding-agent/examples/extensions/plan-mode/README.md

@@ -0,0 +1,65 @@
+# Plan Mode Extension
+
+Read-only exploration mode for safe code analysis.
+
+## Features
+
+- **Read-only tools**: Restricts available tools to read, bash, grep, find, ls, question
+- **Bash allowlist**: Only read-only bash commands are allowed
+- **Plan extraction**: Extracts numbered steps from `Plan:` sections
+- **Progress tracking**: Widget shows completion status during execution
+- **[DONE:n] markers**: Explicit step completion tracking
+- **Session persistence**: State survives session resume
+
+## Commands
+
+- `/plan` - Toggle plan mode
+- `/todos` - Show current plan progress
+- `Shift+P` - Toggle plan mode (shortcut)
+
+## Usage
+
+1. Enable plan mode with `/plan` or `--plan` flag
+2. Ask the agent to analyze code and create a plan
+3. The agent should output a numbered plan under a `Plan:` header:
+
+```
+Plan:
+1. First step description
+2. Second step description
+3. Third step description
+```
+
+4. Choose "Execute the plan" when prompted
+5. During execution, the agent marks steps complete with `[DONE:n]` tags
+6. Progress widget shows completion status
+
+## How It Works
+
+### Plan Mode (Read-Only)
+- Only read-only tools available
+- Bash commands filtered through allowlist
+- Agent creates a plan without making changes
+
+### Execution Mode
+- Full tool access restored
+- Agent executes steps in order
+- `[DONE:n]` markers track completion
+- Widget shows progress
+
+### Command Allowlist
+
+Safe commands (allowed):
+- File inspection: `cat`, `head`, `tail`, `less`, `more`
+- Search: `grep`, `find`, `rg`, `fd`
+- Directory: `ls`, `pwd`, `tree`
+- Git read: `git status`, `git log`, `git diff`, `git branch`
+- Package info: `npm list`, `npm outdated`, `yarn info`
+- System info: `uname`, `whoami`, `date`, `uptime`
+
+Blocked commands:
+- File modification: `rm`, `mv`, `cp`, `mkdir`, `touch`
+- Git write: `git add`, `git commit`, `git push`
+- Package install: `npm install`, `yarn add`, `pip install`
+- System: `sudo`, `kill`, `reboot`
+- Editors: `vim`, `nano`, `code`