🤖 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/CLAUDE.md

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+---
+status: generated
+domain: <Domain Name>
+summary: <One-line summary of what this domain covers.>
+covers:
+  - <path/to/main/source/>
+  - <path/to/tests/>
+related: []
+status: active
+---
+# <Domain Name>
+
+<!-- Replace this heading and fill the frontmatter during genesis orientation.
+     This file is the index — keep it under ~60 lines. It is auto-loaded by
+     the harness whenever any file in this folder is read. -->
+
+<!-- One paragraph: scope, boundaries, what is and is not this domain. -->
+
+## Must-read
+
+Every genesis, before answering anything. Keep this list to three or four entries.
+
+- `handoff.md` — where the previous adept left things. Always first.
+- `key-files.md` — the files that matter, what each does, where the seams are.
+<!-- - `conventions.md` — rules here that differ from the project-wide ones. -->
+
+## Good-to-read-if
+
+<!-- State the condition for each entry. -->
+
+<!-- - `architecture.md` — if asked how the pieces fit, or where a new thing should go. -->
+<!-- - `gotchas.md` — if asked why something behaves oddly. -->
+- `journal/` — if `handoff.md` points at a specific entry, or a question touches a dead end noted there. Do not read the whole journal.
+
+## Open questions
+
+Carried forward by predecessors. Try to resolve one before adding one.
+
+<!-- - "Why does FooService bypass the standard auth middleware?" (since YYYY-MM-DD) -->

docs/workpad/adepts/_template/handoff.md

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+---
+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>'