feat(release)!: rename plugin and repo from coord to repo-mailbox

The old name said that something was coordinated, but not what the thing
was. The new one names what it is, reusing the vocabulary the code and
docs already use throughout: mailbox, inbox, archive, broadcast.

Renamed: Forgejo repo (open/coord -> open/repo-mailbox, old URL
redirects), plugin manifest name, package name, README title and badge,
CLAUDE.md heading and release command.

BREAKING CHANGE: the skill is invoked as /repo-mailbox:coord-send rather
than /coord:coord-send, and the plugin must be reinstalled under its new
name.

Deliberately unchanged: the CLI (coord-send.sh, coord-inbox.sh,
coord-done.sh, coord-selftest.sh), the skill name coord-send, the mailbox
root ~/.claude/coord/, and CLAUDE_COORD_DIR. Those name the transport
protocol, not the product; renaming them would migrate live mailbox data
and break message history in every participating repo for no gain.

Selftest unchanged at 70/70 -- the engine was not touched.

CLAUDE.md

@@ -1,4 +1,9 @@
-# coord
+# repo-mailbox
+
+Renamed from `coord` in v0.3.0. The plugin/repo is `repo-mailbox`; the CLI
+(`coord-send.sh`, `coord-inbox.sh`, `coord-done.sh`), the mailbox root
+(`~/.claude/coord/`) and `CLAUDE_COORD_DIR` deliberately kept their names —
+they are the transport protocol, not the product.
 
 ## Context
 
@@ -46,7 +51,8 @@ Version must agree across: `.claude-plugin/plugin.json`, `package.json`,
 README version badge, `skills/coord-send/SKILL.md` frontmatter, git tag
 `vX.Y.Z`, and the catalog `ref` in
 `ktg-plugin-marketplace/catalog/.claude-plugin/marketplace.json`. Release via
-the catalog's `scripts/release-plugin.mjs coord` (tag + ref bump together);
+the catalog's `scripts/release-plugin.mjs repo-mailbox` (tag + ref bump
+together);
 verify with `scripts/check-versions.mjs`. Never hand-edit a ref.
 
 Two things that script does that its dry-run label does not suggest: