feat(governance): land authoritative PROJECT_CHARTER.md (#987)

Introduce the project charter as the ultimate source of truth for purpose,
operating model, principles, non-goals, success criteria, and change-control.
Point AGENTS.md at the charter without replacing it.

Closes #987

AGENTS.md

@@ -0,0 +1,23 @@
+# Agent Rules
+
+## Project charter (authoritative)
+
+Before inspecting work, selecting a task, claiming work, implementing, reviewing,
+approving, or merging, read the full project charter:
+
+**`PROJECT_CHARTER.md`**
+
+That file is the ultimate source of truth for purpose, direction, principles,
+non-goals, success criteria, and change-control. This file only points to it;
+do not treat this file, session memory, or prompts as a substitute for the charter.
+
+If proposed work would change the project’s fundamental direction or any
+non-negotiable principle in the charter, stop and present the change to the
+human maintainer for an explicit decision recorded in Gitea.
+
+## Issue-first workflow
+
+- Never fix code directly. A Gitea issue must be created first, and all fix work happens under that issue (branch, PR, review, merge) per the canonical workflow.
+- No implementation, remediation, refactor, operational change, or charter amendment may begin without an open Gitea issue authorizing that work.
+- Investigation may prepare or validate an issue; it must not silently become implementation.
+- Every implementation PR must reference its governing issue.