docs(agents): add a core rule that points to the module-boundaries rule (#20598)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - `AGENTS.md` section 5 holds the core engineering rules that apply to
every change
> - The section had no rule about which module owns a behavior
> - The repository already has a repository-wide rule for this,
`rules/module-boundaries.mdc`
> - Agents see that rule only as one line in the list of rule headers
> - This pull request adds a core rule that states the principle and
links to that rule
> - The benefit is that module ownership has the same weight as the
other core rules

## Linked Issues or Issue Description

Refs #20585

**Issue type**

Missing contributor documentation.

**Where is the issue?**

`AGENTS.md` section 5.

**What's wrong?**

- The core engineering rules did not say that provider-specific behavior
stays with the module that owns it.
- The core engineering rules did not point to
`rules/module-boundaries.mdc`.

**Suggested fix**

Add a core rule that states the principle and links to the existing
rule.

## What Changed

- Added rule 4 to `AGENTS.md` section 5, "Keep behavior with its owning
module". The rule links to `rules/module-boundaries.mdc`.
- An earlier commit on this branch added a copy of the pstack
`principle-boundary-discipline` skill. A later commit removed it. The
net change is `AGENTS.md` only.

## Verification

- Open `AGENTS.md` section 5 and follow the link in rule 4.
- Run `git diff origin/master --stat`. Only `AGENTS.md` changes.
- No source code changed. No test suite ran locally.

## Risks

- Low risk. The change is documentation only.

## Model Used

- Claude (Anthropic), in Claude Code with tool use.
- `claude-fable-5-2`.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [ ] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

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

---------

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

AGENTS.md

@@ -96,6 +96,9 @@ preflight", "Start Dev", "Quick Health Checks", and "Database in Dev" in
    - Budget hard-stop auto-pause behavior
    - Activity logging for mutating actions
 
+4. **Keep behavior with its owning module.**
+   Provider protocols, payload parsing, and feature-specific sequencing belong to the module that owns them. Shared code uses narrow, normalized contracts. Explain any substantive change outside the owning module. See [`rules/module-boundaries.mdc`](rules/module-boundaries.mdc).
+
 ## 6. Database Changes
 
 Follow "Schema change workflow" in `doc/DATABASE.md`: edit the schema, run