docs: clear the remaining error-level vale lints

Per #4128 (mara: allow-everywhere false positives go in a central
list, otherwise fix in source). Testing surfaced better fixes than
the plan posted on the issue:

- 5x Microsoft.Contractions 'that is' idiom false positives: adding
  the missing comma ("that is, ...") both reads better and satisfies
  the rule's own negative-lookahead, so no suppression is needed at
  all. Fixed in docs/integrations/forge.md, docs/tools/forge.md,
  docs/tools/hivectl.md, docs/web-ui/dashboard.md, and
  swarmctl-cli.md's generated source (swarmctl/src/main.rs, doc
  comment regenerated via markdown-docs).
- persistence.md's 'is not' matching inside 'is nothing': reworded to
  'there'\''s nothing' rather than add any exception -- dodges the trap
  and is a genuine contraction besides.
- ca.md's 'it is' matching inside the already-correct 'it isn'\''t':
  tried a central .vale.ini TokenIgnores entry first per the
  allow-everywhere framing, but testing against the real file (not
  just a synthetic snippet) found it silently fails to suppress
  whenever markdown emphasis syntax appears earlier in the same
  file -- an offset-drift bug in how Vale applies TokenIgnores, not
  a config mistake. Reworded to "it'\''s not" instead, same fix
  shape as persistence.md.
- config.md's 3 genuine Microsoft.Avoid 'backend' exceptions (already
  flagged and accepted on #4139 -- an actually-pluggable LLM API
  provider, matching the nix option's own name, not one internal
  system to name): scoped inline vale suppression around just that
  section, since this one really is context-specific rather than a
  rule bug.

Verified: fresh 'vale docs/ --minAlertLevel=error' is 0 errors AND
0 warnings (was 10 errors). nix fmt 0 changed beyond the edits
themselves. pre-push lints (tracker-tag/comment-block/doc-pointer)
clean. cargo clippy -p swarmctl -- -D warnings clean. Diffed the
regenerated swarmctl-cli.md against the old copy to confirm only
the intended line moved.

docs/agent-lifecycle/persistence.md

@@ -479,7 +479,7 @@ so it survives restarts and redeploys.
   with. **Keep it.** It's handed to Forgejo when swarm-controller registers a hook,
   so replacing the file means every subsequent delivery fails
   verification until the hook is re-registered with the new value. It's
-  generated automatically on first start; there is nothing to configure.
+  generated automatically on first start; there's nothing to configure.
 
 If the file is unreadable at startup the daemon still starts and logs
 `webhook secret unavailable`; the webhook endpoint then answers 503