docs: fix capitalization losses + one text corruption from the contractions pass

Review pass on top of the previous commit (85a94671) turned up three
real issues its own vale-only verification didn't catch:

- 'has not' -> 'hasn't' fired inside 'has nothing', corrupting
  docs/swarm/README.md into 'hasn'thing to say upward'. Restored
  'has nothing'.
- ~44 hits where the original text was genuinely sentence-initial
  ('It is', 'That is', 'They are', 'What is', 'Do not', ...) got
  contracted in lowercase, silently de-capitalizing the start of a
  sentence. Checked each one's real surrounding text and capitalized
  the ones that needed it.
- Confirmed (by reading the actual text, not just trusting vale) that
  the 3 remaining 'is not'/'has not'/'it is' hits after those fixes are
  vale's own false positives: it matches these as a bare substring with
  no trailing word boundary, so it also fires on 'is nothing', 'has
  nothing', and the already-correct 'it isn't'. Left those alone --
  there's no real issue in the text.

Re-verified with a fresh vale run: the remaining 17 hits are exactly
the union of the 14 legitimate ALL-CAPS/bold-emphasis skips from the
first commit and these 3 false positives. Full accounting in
/agents/iris/state/haiku-batches/contractions-report.md.

docs/agent-lifecycle/persistence.md

@@ -476,9 +476,9 @@ Only present on the one host running
 so it survives restarts and redeploys.
 
 - `webhook-secret` — the HMAC key the swarm's forge webhooks are signed
-  with. **Keep it.** it's handed to Forgejo when a hook is registered,
+  with. **Keep it.** It's handed to Forgejo when a hook is registered,
   so replacing the file means every subsequent delivery fails
-  verification until the hook is re-registered with the new value. it's
+  verification until the hook is re-registered with the new value. It's
   generated automatically on first start; there is nothing to configure.
 
 If the file is unreadable at startup the daemon still starts and logs