docs: update Antigravity porting guidance

[anhtnt90dev]

Who is submitting this PR? (required)

Field	Value
Your model + version	GPT-5 (exact model ID not exposed by Codex CLI)
Harness + version	Codex CLI 0.137.0 on Windows/PowerShell
All plugins installed	Browser, Documents, Presentations, Sites, Spreadsheets, Superpowers
Human partner who reviewed this diff	anhtnt90dev approved the complete diff in Codex chat

What problem are you trying to solve?

docs/porting-to-a-new-harness.md still described Antigravity as if it used a repo-local .antigravity-plugin/install.sh scaffold that generated a contextFileName bootstrap file.

That is stale after #1657. The merged Antigravity support installs the existing repository directly:

agy plugin install https://github.com/obra/superpowers

The #1657 commit message explicitly says no scaffold, installer, or generated context file is needed. The current tree also has no .antigravity-plugin/ directory; Antigravity support consists of the README install command, the antigravity-tools.md mapping, and tests/antigravity/.

What does this PR change?

Updates the porting guide so Antigravity is described as a direct plugin install that runs the existing session-start hook. Removes the stale .antigravity-plugin/install.sh / generated context-file example from the generic context-file guidance.

Is this change appropriate for the core library?

Yes. This is documentation for the core repo's own harness-porting guide, and it keeps that guide aligned with the Antigravity support that already landed in #1657.

What alternatives did you consider?

I considered adding an .antigravity-plugin/ scaffold to match the guide, but that would contradict the accepted #1657 implementation, which intentionally avoided a scaffold. I also considered removing Antigravity from the guide entirely, but it is a useful current reference integration; the guide just needs to describe its actual shape.

Does this PR contain multiple unrelated changes?

No. It only updates Antigravity references in docs/porting-to-a-new-harness.md.

Existing PRs

• I have reviewed all open AND closed PRs for duplicates or prior art
• Related PRs: feat: add Antigravity CLI (agy) support #1657, docs: add 'Porting Superpowers to a New Harness' guide #1656, feat: add Antigravity CLI plugin support #1583

#1656 added the porting guide before the final Antigravity implementation landed. #1657 added Antigravity support and states that no scaffold/installer/generated context file is needed. #1583 was a closed prior Antigravity attempt. This PR only reconciles the guide with the merged #1657 implementation.

Environment tested

Harness (e.g. Claude Code, Cursor)	Harness version	Model	Model version/ID
Codex CLI on Windows/PowerShell	0.137.0	GPT-5	GPT-5 (Codex session; exact model ID not exposed by harness)

Authoring environment disclosure: generated with GPT-5 in Codex CLI 0.137.0. Installed Codex plugins available in the session: Browser, Documents, Presentations, Sites, Spreadsheets, Superpowers.

New harness support (required if this PR adds a new harness)

Not applicable. This PR does not add a new harness.

Clean-session transcript for "Let's make a react todo list"

Not applicable. This PR does not add a new harness.

Evaluation

• What was the initial prompt you (or your human partner) used to start the session that led to this change?
  • "hãy tập trung vào repo https://github.com/obra/superpowers để xem có thể contributor được gì không ? dùng account anhtnt90dev để thực hiện"
• How many eval sessions did you run AFTER making the change?
  • 0 LLM eval sessions. This is a docs-only change and does not alter skill or harness behavior.
• How did outcomes change compared to before the change?
  • Before: the porting guide pointed contributors at .antigravity-plugin/install.sh and a generated context file path that do not exist in the accepted Antigravity implementation.
  • After: the guide describes the direct agy plugin install https://github.com/obra/superpowers flow and no longer references the nonexistent Antigravity scaffold.

Verification run after the change:

git diff --check HEAD~1..HEAD
# exit 0

rg -n "\.antigravity-plugin|\.antigravity-plugin/install\.sh|context : ANTIGRAVITY\.md" docs/porting-to-a-new-harness.md
# no matches

bash tests/antigravity/run-tests.sh
# All Antigravity tests passed

Rigor

• If this is a skills change: I used superpowers:writing-skills and completed adversarial pressure testing (paste results below)
• This change was tested adversarially, not just on the happy path
• I did not modify carefully-tuned content (Red Flags table, rationalizations, "human partner" language) without extensive evals showing the change is an improvement

This is not a skills change. No skill text, prompts, red flags, rationalization lists, or behavior-shaping content were modified.

Human review

• A human has reviewed the COMPLETE proposed diff before submission