docs: add GitHub SSH authentication guidance (#94360)
### What?
Add GitHub SSH authentication guidance to `AGENTS.md` for
user-configured SSH agents and key providers.
### Why?
Remote operations can fail when an SSH agent or key provider is
unavailable or locked. The guidance prevents unrequested authentication
workarounds and directs agents to involve the user when signing fails.
### How?
Document recognized SSH signing failure symptoms, the required
stop-and-ask behavior, and a lightweight preflight recommendation before
force-pushes or stack rebases that may hydrate partial-clone objects.
### Verification
- `pnpm prettier --with-node-modules --ignore-path .prettierignore
--write AGENTS.md`
- `git diff --cached --check`
- `pnpm build-all`
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