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/README.md

@@ -58,7 +58,7 @@ declarations.
   the PAT injected?** → [`integrations/github.md`](integrations/github.md)
   (operator content up top; the `gh`/git-push + notification-poller
   mechanics are in a collapsed "Implementation" section at the bottom).
-- **What is `/knowledge`? How does the hive-wide knowledge repo sync,
+- **What's `/knowledge`? How does the hive-wide knowledge repo sync,
   and how do I contribute a document?** → [`integrations/knowledge.md`](integrations/knowledge.md).
 - **What does `hivectl` do? Provisioning, gateway users, container
   shells?** → [`tools/hivectl.md`](tools/hivectl.md) (the curated guide);