[opt](pr-template) add new pr template - #42999

### What problem does this PR solve?
Problem Summary:

What you see now is the new PR template.
Every contributor and reviewer should take filling out the PR template
seriously.

### What problem does this PR solve?
Problem Summary:

What you see now is the new PR template.
Every contributor and reviewer should take filling out the PR template
seriously.

…it messages (#70175)

### Background

Every PR against `master` starts from
`.github/PULL_REQUEST_TEMPLATE.md`, and the root `AGENTS.md` tells
coding agents to write commit messages in the same layout. The current
template asks for a problem summary, a release note, and checkboxes for
tests, behavior changes, and documentation. Doris merges PRs by
squashing, so the description becomes the commit message on `master`.

### What problem does this PR solve?

Issue Number: None
Related PR: #42999 (introduced the current template), #70123 (a
description written in this layout)
PR Type: Build/CI/Chore

Problem Summary:

The current template leaves out what reviewers most often have to ask
for: the context of the module, why the change is needed and why this
approach, why existing tests missed a bug, and what was verified. Many
PRs leave its checkboxes unticked. This PR replaces it with a layout
that opens with Background, has required fields for each PR type, asks a
diff of more than about 300 changed lines to open Details with a Change
map, and adds Testing, optional Notes for Reviewers, a pre-submission
status, and an AI disclosure. Commit messages written by `AGENTS.md`
rules follow the same layout.

### Details

- **Affected:** `.github/PULL_REQUEST_TEMPLATE.md`, which every new PR
description starts from, and the commit and worktree rules in the root
`AGENTS.md`. No build scripts, CI jobs, or workflows change;
`title-checker.yml` accepts the same `[type](scope)` titles as before.
- **Developer and release impact:** Authors fill in more sections, with
the required fields of each PR type listed in a comment under Details;
the reviewer checklist is gone. Coding agents write commit messages in
the new layout, and from a worktree they may keep PR handoff files in
`${DORIS_REPO}/.git/pr-handoff/`. Releases are unaffected.
- **License and third-party:** None.
- **Rollback:** Revert this PR; nothing else depends on the template.

### Testing

- `check_pr.py`, the checker of the `doris-pr-description` skill (not
part of this PR; see Notes for Reviewers), reports the unfilled template
as not ready, with 17 errors that include the empty Background, Problem
Summary, Testing, and Release note and every blank answer line. Its
eleven bundled examples, one or two per PR type and written in this
layout, pass it, and so does this description.
- The description of #70123, written in this layout, passes the same
checker, with one warning that its prose is longer than the limit.

### Notes for Reviewers

- Two commits: the template and the `AGENTS.md` rules, then guidance for
the Change map and three refined comments. With the HTML comments
removed, the second commit changes nothing.
- `AGENTS.md` also drops the second copy of the paragraph on which files
a commit may contain.
- The new `AGENTS.md` text names the `develop-with-evidence` and
`doris-pr-description` skills, which are not in this repository yet.
They are to be added to
[apache/doris-skills](https://github.com/apache/doris-skills), which a
later PR includes here as a git submodule.

### Release note

None

### Check List (For Author)

- Behavior changed: No
- Does this need documentation: No
- Pre-submission status: Ready

### AI Disclosure

- Tool: Claude Code (Claude Opus 5.5)
- Generated: the template, the `AGENTS.md` text, the commit messages,
and this description
- Human review: The author reviewed all of the changes line by line.