docs(adr): supersede ADR-0007, plan OneDev migration

This repo's own hosting, issue tracking, and pull requests move from Gitea
to OneDev (ADR-0029), keeping the Gitea repo as a read-only archive rather
than deleting it. plugins/gitea is unaffected — it continues to ship as a
marketplace product regardless of what this repo hosts itself on.

Records the full execution plan (prerequisites, mirror/issue/PR/release
phases, verification checklist) and updates CONTEXT.md's Issue entry and
AGENTS.md's source-of-truth line to name OneDev instead of Gitea.

ADR: 0029
Refs: ADR-0007
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GGCPJBXPLJP5FprL4C8nu3

CONTEXT.md

@@ -133,8 +133,9 @@ than to enumerate siblings. Detail: `factory-audit/references/skill-description-
 _Avoid_: overlap, similar skill
 
 **Issue**:
-The cross-provider term for a tracked unit of work. Gitea is this repo's canonical tracker
-(ADR-0007), but skills say "linked issue" generically rather than naming a provider.
+The cross-provider term for a tracked unit of work. OneDev is this repo's canonical tracker
+(ADR-0007, superseded by ADR-0029), but skills say "linked issue" generically rather than naming
+a provider.
 _Avoid_: ticket, card, task
 
 **Family prefix**: