docs(agents): cross-linking house rule + mastodon post tool [#743][#441]

Clickable-refs mandate from Charles: ticket comments link commits,
commit bodies link the Redmine comment URL, Discourse links both —
one click between Redmine/Gitea/Discourse (GLPI/CMDB change control
will extend this). Mechanical body-URL check queued under #441.
scripts/mastodon/post.sh: zero-install curl poster; creds land in
~/.creds/mastodon.env (MASTODON_URL/MASTODON_TOKEN, write scope).

scripts/mastodon/post.sh

@@ -0,0 +1,49 @@
+#!/usr/bin/bash
+#
+# post.sh — post a work-update status to Mastodon [#743]
+#
+# Zero-install: uses curl against the Mastodon REST API (host-hygiene
+# compliant — no packages on the host). Credentials come from the
+# centralized store:
+#   ~/.creds/mastodon.env   (MASTODON_URL, MASTODON_TOKEN)
+# Token needs write scope; create via Mastodon → Preferences → Development.
+#
+# Usage:
+#   post.sh [-v public|unlisted|private|direct] "status text (<=500 chars)"
+#
+set -euo pipefail
+
+CREDS="${HOME}/.creds/mastodon.env"
+if [ ! -r "$CREDS" ]; then
+  echo "post.sh: missing $CREDS (needs MASTODON_URL, MASTODON_TOKEN)" >&2
+  exit 2
+fi
+# shellcheck disable=SC1090
+. "$CREDS"
+if [ -z "${MASTODON_URL:-}" ] || [ -z "${MASTODON_TOKEN:-}" ]; then
+  echo "post.sh: MASTODON_URL / MASTODON_TOKEN not set in $CREDS" >&2
+  exit 2
+fi
+
+visibility="public"
+if [ "${1:-}" = "-v" ]; then
+  visibility="$2"
+  shift 2
+fi
+if [ $# -lt 1 ]; then
+  echo "usage: $0 [-v public|unlisted|private|direct] \"status text\"" >&2
+  exit 2
+fi
+
+status="$1"
+len=${#status}
+if [ "$len" -gt 500 ]; then
+  echo "post.sh: status too long ($len chars; hard ceiling 500)" >&2
+  exit 1
+fi
+
+curl -sS --max-time 30 -X POST "$MASTODON_URL/api/v1/statuses" \
+  -H "Authorization: Bearer $MASTODON_TOKEN" \
+  --data-urlencode "status=$status" \
+  --data-urlencode "visibility=$visibility" |
+  jq -r '"posted id=\(.id) url=\(.url) visibility=\(.visibility)"'