feat: add multi-tool integration scripts and converted agent files

Adds support for installing The Agency agents across 7 agentic coding
tools via two scripts:

scripts/convert.sh
  - Reads all 61 agents and converts them into tool-specific formats
  - Supports: Antigravity, Gemini CLI, OpenCode, Cursor, Aider, Windsurf
  - Shellcheck-clean, pure bash, no external deps
  - Run: ./scripts/convert.sh [--tool <name>]

scripts/install.sh
  - Interactive terminal UI with auto-detection of installed tools
  - Pre-selects detected tools, toggle by number or bulk commands
  - Falls back gracefully in CI / non-interactive mode
  - Cross-platform: Linux, macOS (bash 3.2+), Windows Git Bash / WSL
  - Run: ./scripts/install.sh [--tool <name>] [--no-interactive]

integrations/ (generated, committed for reference)
  - antigravity/   SKILL.md per agent (61 files)
  - gemini-cli/    SKILL.md per agent + gemini-extension.json
  - opencode/      .md agent files for .opencode/agent/
  - cursor/        .mdc rule files for .cursor/rules/
  - aider/         single CONVENTIONS.md (all agents combined)
  - windsurf/      single .windsurfrules (all agents combined)
  - claude-code/   README only (agents copied directly from repo root)
  - README.md      full usage + tool-specific instructions

README.md
  - Added Multi-Tool Integrations section with supported tools table,
    quick-start guide, per-tool expandable instructions, and updated
    roadmap marking integrations as complete

integrations/claude-code/README.md

@@ -0,0 +1,31 @@
+# Claude Code Integration
+
+The Agency was built for Claude Code. No conversion needed — agents work
+natively with the existing `.md` + YAML frontmatter format.
+
+## Install
+
+```bash
+# Copy all agents to your Claude Code agents directory
+./scripts/install.sh --tool claude-code
+
+# Or manually copy a category
+cp engineering/*.md ~/.claude/agents/
+```
+
+## Activate an Agent
+
+In any Claude Code session, reference an agent by name:
+
+```
+Activate Frontend Developer and help me build a React component.
+```
+
+```
+Use the Reality Checker agent to verify this feature is production-ready.
+```
+
+## Agent Directory
+
+Agents are organized into divisions. See the [main README](../README.md) for
+the full roster of 61 specialists.