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/opencode/README.md

@@ -0,0 +1,58 @@
+# OpenCode Integration
+
+OpenCode uses the same agent format as Claude Code — `.md` files with YAML
+frontmatter stored in `.opencode/agent/`. No conversion is technically
+needed, but this integration packages the agents into the correct directory
+structure for drop-in use.
+
+## Install
+
+```bash
+# Run from your project root
+cd /your/project
+/path/to/agency-agents/scripts/install.sh --tool opencode
+```
+
+This creates `.opencode/agent/<slug>.md` files in your project directory.
+
+## Activate an Agent
+
+In OpenCode, reference an agent by its name or description:
+
+```
+Use the Frontend Developer agent to help build this component.
+```
+
+```
+Activate the Reality Checker agent and review this PR.
+```
+
+You can also select agents from the OpenCode UI's agent picker.
+
+## Agent Format
+
+OpenCode agents use the same frontmatter as Claude Code:
+
+```yaml
+---
+name: Frontend Developer
+description: Expert frontend developer specializing in modern web technologies...
+color: cyan
+---
+```
+
+## Project vs Global
+
+Agents in `.opencode/agent/` are **project-scoped**. To make them available
+globally across all projects, copy them to your OpenCode config directory:
+
+```bash
+mkdir -p ~/.config/opencode/agent
+cp integrations/opencode/agent/*.md ~/.config/opencode/agent/
+```
+
+## Regenerate
+
+```bash
+./scripts/convert.sh --tool opencode
+```