v0.3.0: rename app Daybook -> AgentFeed

Removes the "Daybook" name from everything user- and agent-facing, since no
agents are configured yet and this is the cheap moment to do it.

- UI brand, docs, README, CLAUDE.md -> AgentFeed.
- Ingest env vars DAYBOOK_URL/TOKEN/SOURCE/DATA_DIR/BIND_ADDR/HOST_DATA_DIR/
  KEY_PROVISIONING_API -> AGENTFEED_* (token *values* unchanged).
- Skill: skills/daybook-posting -> skills/agentfeed-posting, daybook_post.py
  -> agentfeed_post.py (+ scripts/ wrapper and its test).
- static/daybook.css -> static/agentfeed.css; localStorage daybook.* ->
  agentfeed.*; templatetag daybook_extras -> agentfeed_extras; multipart
  boundaries; json/csv viewer element ids; daybook_exception_handler ->
  agentfeed_exception_handler; compose service/container -> agentfeed.
- Key-provisioning API now OFF by default (independent of DEBUG); enable with
  AGENTFEED_KEY_PROVISIONING_API=1 on test installs only.
- Posting helper now points at Settings -> API Keys.

Deferred on purpose (a later, separate step): the Django package
`daybook_project`, the `/opt/daybook` install path, and the Forgejo repo name.
The sibling AGENT_DAYBOOK__LLM_TEST name is intentionally untouched.

Full suite: 129 passing; live server verified serving under the new name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/artifacts/settings_base.html

@@ -6,12 +6,12 @@
   <title>Settings — AgentFeed</title>
   <link rel="icon" type="image/png" href="/static/favicon.png">
   <link rel="apple-touch-icon" href="/static/apple-touch-icon.png">
-  <link rel="stylesheet" href="/static/daybook.css">
+  <link rel="stylesheet" href="/static/agentfeed.css">
   <script>
     // Set the theme before first paint so dark-mode users never see a light flash.
     (function () {
       try {
-        if (localStorage.getItem("daybook.theme") === "dark") {
+        if (localStorage.getItem("agentfeed.theme") === "dark") {
           document.documentElement.setAttribute("data-theme", "dark");
         }
       } catch (e) {}