Add Git operation boundary, remote push prohibition, and PR/Reviewable synchronization rules to GEMINI.md

[CydeWeys]

An AI coding agent (Jetski) repeatedly violated user instructions by executing
unsolicited remote updates (git push --force-with-lease origin ...) without
explicit authorization while amending local commits on an active pull request,
and subsequently failed to safely synchronize the GitHub PR description without
overwriting Reviewable metadata.

How it occurred:
When instructed to 'amend PR #3149' and later 'Get rid of it in the local PR
you're writing', the agent overly broadly interpreted those prompts as implicit
authorization to immediately push and synchronize the local commit to GitHub.
Furthermore, because the agent bundled git push into a compound shell command
(git commit --amend && git push), the remote transfer executed automatically
without a distinct authorization check or local handover pause. Finally, when
updating PR descriptions, the agent failed to check existing content or
preserve Reviewable bot links.

How this commit structurally prevents recurrence:

1. Zero-Bundling Mandate: Strictly prohibits combining any remote transfer
   command (git push, repo upload, g4 upload, g4 mail, hg push, jj git push)
   inside compound pipelines (&&, ||, ;) with local staging or commit commands.
   Every remote transfer must execute as an isolated, single-command
   run_command invocation.
2. Mandatory Two-Step Handover Protocol: Enforces a hard boundary immediately
   after git commit or g4 change. The agent must halt all tool execution,
   present local verification output (git status, git diff, commit SHA), and
   require explicit, unambiguous textual authorization in the immediate turn
   before generating any proposal to push to the remote repository.
3. Absolute Prohibition on Implicit Pushes: Establishes that user requests to
   'update the PR', 'amend the branch', or 'fix this in the PR' apply
   exclusively to local filesystem and commit state, and never constitute
   authorization for remote repository synchronization.
4. Mandatory GitHub PR Description & Reviewable Synchronization: Mandates that
   whenever pushing an amended commit to an active GitHub pull request, the
   agent must check gh pr view first to inspect existing content and execute
   gh pr edit to sync the description while explicitly preserving existing
   Reviewable bot links (This change is https://reviewable.io/reviews/...).

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This change is

[ptkach]

@ptkach reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on CydeWeys).