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/integrations/forge.md

@@ -279,7 +279,7 @@ to `closed_at` that `state_change_is_current` returns `true` — so it takes
 the state-change path, dropping its body. Best of both worlds:
 on the merge/close path `forge_notify` fetches the `latest_comment_url` comment and, when
 its `created_at` is strictly **after** the subject's `closed_at`
-(`comment_is_after_close`) — that is it raced the merge rather than being the
+(`comment_is_after_close`) — that is, it raced the merge rather than being the
 pre-merge last comment the subject keeps — append it as a
 `comment by <author>: <excerpt>` block before the meta suffix
 (`fresh_post_close_comment_tail`). The wake carries **both** `[PR merged]`