docs: add Antigravity setup and usage guide (#552)

* docs: add Antigravity setup and usage guide

Addresses #462 — users were confused about Antigravity skills setup.

Adds a comprehensive guide covering:
- Install mapping (ECC → .agent/ directory)
- Directory structure after install
- openai.yaml agent config format
- Managing installs (list, doctor, uninstall)
- Cross-target comparison table
- Troubleshooting common issues
- How to contribute skills with Antigravity support

Also links the guide from the README FAQ section.

* fix: address review feedback on Antigravity guide

- Remove spurious skills/ row from install mapping table, add note
  clarifying .agents/skills/ is static repo layout not installer-mapped
- Fix repair section: doctor.js diagnoses, repair.js restores
- Fix .agents/ → .agent/ path typo in custom skills section
- Clarify 3-step workflow for adding Antigravity skills
- Fix antigravity-project → antigravity in comparison table
- Fix "flatten" → "flattened" grammar in README
- Clarify openai.yaml full nested path structure

* fix: clarify .agents/ vs .agent/ naming and fix Cursor comparison

- Explain that .agents/ (with 's') is ECC source, .agent/ (no 's')
  is Antigravity runtime — installer copies between them
- Fix Cursor Agents/Skills column: Cursor has no explicit agents/skills
  mapping (only rules), changed from 'skills/' to 'N/A'

* fix: correct installer behavior claims and command style

- Fix .agents/ vs .agent/ note: clarify that only rules, commands, and
  agents (no dot) are explicitly mapped by the installer. The dot-prefixed
  .agents/ directory falls through to default scaffold, not a direct copy.
- Fix contributor workflow: remove false auto-deploy claim for openai.yaml.
  Clarify .agents/ is static repo layout, not installer-deployed.
- Fix uninstall command: use direct script call (node scripts/uninstall.js)
  for consistency with doctor.js, repair.js, list-installed.js.

* fix: add missing agents/ step to contributor workflow

Contributors must add an agent definition at agents/ (no dot) for the
installer to deploy it to .agent/skills/ at runtime. Without this step,
skills only exist in the static .agents/ layout and are never deployed.

---------

Co-authored-by: vazidmansuri005 <vazidmansuri005@users.noreply.github.com>

README.md

@@ -835,7 +835,7 @@ Yes. ECC is cross-platform:
 - **Cursor**: Pre-translated configs in `.cursor/`. See [Cursor IDE Support](#cursor-ide-support).
 - **OpenCode**: Full plugin support in `.opencode/`. See [OpenCode Support](#-opencode-support).
 - **Codex**: First-class support for both macOS app and CLI, with adapter drift guards and SessionStart fallback. See PR [#257](https://github.com/affaan-m/everything-claude-code/pull/257).
-- **Antigravity**: Tightly integrated setup for workflows, skills, and flatten rules in `.agent/`.
+- **Antigravity**: Tightly integrated setup for workflows, skills, and flattened rules in `.agent/`. See [Antigravity Guide](docs/ANTIGRAVITY-GUIDE.md).
 - **Claude Code**: Native — this is the primary target.
 </details>