docs: add persistent agent constraints document (issue #100)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/agent-constraints.md

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+# Agent Constraints
+
+This document records **persistent behavioral constraints** for agents working on this repository.
+It is distinct from `docs/requirements-log.md`, which records **project/product requirements**.
+
+## Scope
+
+- Applies to all AI agents and automation that modify this repo.
+- Supplements (does not replace) `docs/agents.md` and `docs/workflow.md`.
+
+## Persistent Rules
+
+1. **Workflow enforcement**
+   - Follow `docs/workflow.md` for all changes.
+   - Create a Gitea issue before any code or documentation change.
+   - Work on a feature branch `feature/issue-{N}-{short-description}` and open a PR.
+   - Never commit directly to `main`.
+
+2. **Document-first routing**
+   - When performing work, consult relevant `docs/` files *before* making changes.
+   - Route decisions to the documented policy whenever applicable.
+   - If guidance conflicts, prefer the stricter/safety-first rule and note it in the PR.
+
+3. **Docs with code**
+   - Any code change must be accompanied by relevant documentation updates.
+   - If no doc update is needed, state the reason explicitly in the PR.
+
+4. **Session-persistent user constraints**
+   - If the user requests that a behavior should persist across sessions, record it here
+     (or in a dedicated policy doc) and reference it when working.
+   - Keep entries short and concrete, with dates.
+
+## Change Control
+
+- Changes to this file follow the same workflow as code changes.
+- Keep the history chronological and minimize rewording of existing entries.
+
+## History
+
+### 2026-02-08
+
+- Always enforce Gitea workflow: issue -> feature branch -> PR before changes.
+- When work requires guidance, consult the relevant `docs/` policies first.
+- Any code change must be accompanied by relevant documentation updates.
+- Persist user constraints across sessions by recording them in this document.