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).

AGENTS.md

@@ -424,6 +424,28 @@ vendor/             Vendored KNELShellFramework
    justification. A script that emits any diagnostic is a protocol violation.
    Non-bash scripts (PHP with `.sh` shebang `#!/usr/bin/php`, etc.) are exempt.
 
+## Cross-linking (NON-NEGOTIABLE)
+
+**Every artifact cross-references its related artifacts with CLICKABLE
+links.** House rule (Charles, 2026-09-02, [#743]) — becomes hard
+requirement once GLPI/CMDB change control is online (ITIL/ITSM ramp):
+
+- Redmine ticket comments link the commits they describe (full Gitea URL,
+  e.g. `https://git.knownelement.com/KNEL/PFVCluster/commit/<sha>`).
+- Commit bodies carry the full URL of the corresponding Redmine comment
+  (`https://projects.knownelement.com/issues/NNN#note-M`) — the "meat"
+  lives in the comment; the commit body points at it.
+- Discourse posts link both the Redmine ticket and the Gitea repo/commit;
+  Redmine tickets and repo docs link their Discourse topics.
+- One click from ANY system reaches the other two. An artifact without its
+  links is not done.
+- When GLPI is online: change requests become the entry point for infra
+  changes and get cross-referenced into Redmine/Discourse/Gitea the same
+  way; CMDB items link their tickets and vice versa.
+
+Mechanical enforcement (rule-engine check for the ticket-comment URL in
+commit bodies when `[#NNN]` is present) is tracked under [#441].
+
 ## TDD & Linting
 
 - **Red/green TDD for all code.** Mandatory (founder 2026-08-27). Interim