docs: add subagent support to README

[Copilot]

The agent: component type (installs Markdown files to <project>/.claude/agents/) was fully implemented but absent from the README.

Changes

• The Solution prose — added agents to the packaged artifact list
• How It Works artifact table — added Agents | <project>/.claude/agents/ | Per-project row
• What's in a Tech Pack? table — added 🤖 agent | Custom subagents row with example

- id: code-reviewer
  description: Code review subagent
  agent:
    source: agents/code-reviewer.md
    destination: code-reviewer.md

Original prompt

We need to update README, the tool now supports subagents but it's not mentioned on readme file

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[Copilot]

Pull request overview

Updates the README to document the already-implemented agent: component type, ensuring the docs accurately reflect that tech packs can install per-project subagents into .claude/agents/.

Changes:

• Add “agents” to the README’s lists of supported/installed artifacts.
• Add an “Agents” row to the “How It Works” artifact location table.
• Add an 🤖 agent row to the “What’s in a Tech Pack?” component type table (with an example).

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