Publish agent specs transactionally

[schickling-assistant]

Why

st2 had multiple declaration writers with independent locking. A presentation edit could race a spec publication, so readers could observe catalog state that no single writer had admitted.

What

• make st2 agent publish the sole policy-free single-Agent-Spec publication boundary
• serialize publication, presentation authoring, and declaration readers through one persistent .st2/catalog-authoring.lock
• use full-catalog validation, input capture digests, compare-and-swap, atomic replacement, and fsync
• retire the experimental compile-agent raw writer instead of keeping a compatibility path around the transaction boundary

The rebase preserves current stable identity, rename/describe, task inventory, presentation projection, eval requester, and presentation repair-cursor behavior.

Verification

• presentation authoring: 10/10, including a causal cross-command lock-contention test
• agent publication: 13/13
• catalog selection: 6/6
• run-loop regressions: 28/28
• validation: 36/36
• invariant references: 1/1
• maintained native authoring guide: green
• exact touched/new-file rustfmt and git diff --check: green

Two current-main NixOS fixture assumptions remain independently red under the local dev shell: clean PATH=/usr/bin:/bin cannot find Git, and a retired-name substring check matches the sentence st2 removes. Neither is changed or repaired here.

Closes #41.

Posted on behalf of @schickling

field	value
agent_name	co1-sage
agent_tool	Codex CLI
agent_tool_version	0.145.0
agent_runtime	Codex CLI 0.145.0
agent_session_lookup	sha256:ad381a588faa911d70bf08ee2ae4305e96d980701e6405897e32fe38c3163d98
tooling_profile	dotfiles@de765ec