Say how to create a stacked PR

The stacked-PR guidance did not say whether you hand `gh stack link`
branches or PR numbers, so it read as though the PRs had to exist first.
Either works, and branches alone are enough: the command pushes them and
opens the PRs it does not find.

Say what linking does to the bases too. It moves each one onto the branch
below, which looks like a misconfigured PR if you are not expecting it,
and CI keeps running throughout.

Signed-off-by: Cristiano Calcagno <ccrisccris@gmail.com>

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8g8qwBARAcvW9MyuKQq8H

Author: cristianoc

AGENTS.md

@@ -292,11 +292,15 @@ The compiler is designed for fast feedback loops and scales to large codebases:
 
 ### Stacked pull requests
 
-When a PR depends on another unmerged PR, create a native GitHub stack with
-`gh stack` rather than only targeting the preceding feature branch. Keep the
-branches linear and in the same repository, and list branches or PRs from
-bottom to top. For existing PRs, use `gh stack link BOTTOM_PR [NEXT_PR...]`,
-then verify that GitHub reports stack metadata and runs CI for every PR.
+When a PR depends on another unmerged PR, make a native GitHub stack. Keep the
+branches linear and in the same repository, then run `gh stack link BOTTOM
+[NEXT...]`, listing the stack bottom to top. Each argument is a branch name or
+a PR number: the command pushes each branch, reuses the PR that already exists
+for it, and opens one where there is none, so branches alone are enough. Open
+the PRs yourself first if you want to write their titles and descriptions.
+
+Linking sets each PR's base to the branch below it, leaving only the bottom PR
+on `master`. CI runs on every PR in the stack.
 
 ### Code Quality