docs: add git workflow guidelines to prevent force pushing (#21207)

When working on PRs, Claude Code was sometimes force pushing to
branches. This adds simple git workflow guidelines that emphasize proper
branch checkout and avoiding force pushes.

## Changes

Added git workflow section to `CLAUDE.md`, `AGENTS.md`, and
`.claude/docs/WORKFLOWS.md` with:
- Instructions to fetch, checkout, and pull before working on PR
branches
- Note to avoid `git push --force` unless explicitly requested

## Examples of force push behavior

Observed in recent PRs:
- PR #21148: 7 commits including merge commit from iterative changes
- PR #21150: 9 commits with multiple documentation iterations
- PR #21182: 4 commits with iterative fixes
- Force update on `feat/add-tasks-template-flag` branch:
`9bf7980b9...f98cf44f7`

The guidelines now make it clear to check out branches properly and push
normally.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

.claude/docs/WORKFLOWS.md

@@ -121,6 +121,20 @@
 - Use `testutil.WaitLong` for timeouts in tests
 - Always use `t.Parallel()` in tests
 
+## Git Workflow
+
+### Working on PR branches
+
+When working on an existing PR branch:
+
+```sh
+git fetch origin
+git checkout branch-name
+git pull origin branch-name
+```
+
+Then make your changes and push normally. Don't use `git push --force` unless the user specifically asks for it.
+
 ## Commit Style
 
 - Follow [Conventional Commits 1.0.0](https://www.conventionalcommits.org/en/v1.0.0/)