🤖 Add Zathras agent and initialize Spire for reliquary

Scaffold .claude/spire/project.md with reliquary-specific facts,
create adept templates at docs/workpad/adepts/, build the Zathras
agent and skill (Spire's self-maintenance coordinator), and wire
/assistant as a local alias for spire:zathras.

docs/workpad/adepts/_template/handoff.md

@@ -0,0 +1,25 @@
+---
+status: generated
+updated: <YYYY-MM-DD-HH>
+session: <path to current session journal entry>
+branch: <current branch or worktree>
+verified-at: <commit SHA when solid docs were last checked against code>
+---
+# Handoff — <Domain Name>
+
+## State
+<!-- Where things stand. What is documented and verified, what is not. -->
+
+## In flight (experiences, not solid)
+<!-- Branch observations, unmerged work. These stay here until the code is on main. -->
+
+## Open threads
+<!-- Unfinished investigations, pending questions, next steps. -->
+
+## Successor: read first
+1. This file.
+2. `key-files.md` — most likely to have drifted.
+<!-- 3. `journal/<entry>.md` if a specific branch is still open. -->
+
+## Drift check
+    git log --oneline <verified-at>..HEAD -- '<path1>' '<path2>'