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/antigravity/README.md

@@ -0,0 +1,49 @@
+# Antigravity Integration
+
+Installs all 61 Agency agents as Antigravity skills. Each agent is prefixed
+with `agency-` to avoid conflicts with existing skills.
+
+## Install
+
+```bash
+./scripts/install.sh --tool antigravity
+```
+
+This copies files from `integrations/antigravity/` to
+`~/.gemini/antigravity/skills/`.
+
+## Activate a Skill
+
+In Antigravity, activate an agent by its slug:
+
+```
+Use the agency-frontend-developer skill to review this component.
+```
+
+Available slugs follow the pattern `agency-<agent-name>`, e.g.:
+- `agency-frontend-developer`
+- `agency-backend-architect`
+- `agency-reality-checker`
+- `agency-growth-hacker`
+
+## Regenerate
+
+After modifying agents, regenerate the skill files:
+
+```bash
+./scripts/convert.sh --tool antigravity
+```
+
+## File Format
+
+Each skill is a `SKILL.md` file with Antigravity-compatible frontmatter:
+
+```yaml
+---
+name: agency-frontend-developer
+description: Expert frontend developer specializing in...
+risk: low
+source: community
+date_added: '2026-03-08'
+---
+```