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>

AGENTS.md

@@ -103,6 +103,18 @@ app, err := api.Database.GetOAuth2ProviderAppByClientID(ctx, clientID)
 
 ### Full workflows available in imported WORKFLOWS.md
 
+### Git Workflow
+
+When working on existing PRs, check out the branch first:
+
+```sh
+git fetch origin
+git checkout branch-name
+git pull origin branch-name
+```
+
+Don't use `git push --force` unless explicitly requested.
+
 ### New Feature Checklist
 
 - [ ] Run `git pull` to ensure latest code