docs: harden conversation surface against external interaction

docs/adrs/0001-conversation-surface-hardening.md

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+# ADR-0001: Conversation Surface Hardening
+
+## Status
+
+accepted
+
+## Date
+
+2026-05-17
+
+## Context
+
+The repository's issue, pull-request, and discussion surfaces accept
+unstructured text from arbitrary authors. That text is consumed both by
+human maintainers and by agent-assisted workflows that read historical
+threads as context when responding to current tasks. Unrestricted
+authorship on these surfaces is a path for external input to influence
+those downstream consumers.
+
+The non-functional security requirement is that input on these surfaces
+originates only from vetted authors. The mechanism, scope, and lifetime
+of that restriction are operational concerns and are deliberately not
+recorded in this ADR or in the tracking issue.
+
+## Decision
+
+Issue, pull-request, and discussion authorship on this repository is
+restricted to vetted authors. Past content authored outside the vetted
+set is reviewed and curated where appropriate.
+
+## Consequences
+
+- External authors cannot open or comment on issues, pull requests, or
+  discussions on this repository without prior vetting.
+- Maintainers and approved contributors are unaffected.
+- Existing historical content is preserved except where curation
+  required its removal.
+- Operational details of the mechanism live outside the repository and
+  are not subject to ADR change-control.