docs: fix write-good.So/ThereIs/Weasel lint findings

Fixes the "obvious ones first" slice of #4042 (mara: do the obvious
ones first) -- 81 hits across write-good.So, write-good.ThereIs, and
write-good.Weasel, all in docs/. Each is a genuine sentence rewrite
(lead with the real subject instead of "There is/are", drop a
sentence-initial "So ", replace a vague intensifier), not a blind
regex substitution -- read every hit in its real file context before
touching it.

3 of the 81 hits were in CI-generated CLI docs (docs/tools/{hivectl,
swarmctl,forge}-cli.md) -- fixed at the clap #[arg(...)]/doc-comment
source in hivectl/src/cli.rs, swarmctl/src/main.rs, and
hive-forge/src/verbs/repo_add_collaborator.rs, then regenerated via
each crate's `markdown-docs` subcommand so CI's freshness check stays
green.

Verified: fresh vale re-run shows 0 remaining So/ThereIs/Weasel hits
and no new hits introduced (983->982, exactly the one incidental fix
this pass also picked up at docs/scheduler/observability.md:48).
cargo fmt --check and clippy clean on the three touched crates.

Remaining write-good backlog (Passive: 726, TooWordy: 207) is
judgment-heavy and left for a follow-up slice of #4042, not bulk-
rewritten here.

docs/web-ui/design-guide.md

@@ -16,7 +16,7 @@ repeated here.
 - **Frosted / tinted glass** where it fits — translucency over flat
   panels, not a hard rule for every surface.
 - **Playful use of the multiple accent colours**, without tipping into
-  visual noise — the base16 palette has several accent slots
+  visual noise — the base16 palette has multiple accent slots
   (`--purple`, `--cyan`, `--pink`, …); use more than one where it adds
   distinction, not decoration for its own sake.
 - **Whimsy** — small, delightful touches are welcome (the per-hive
@@ -110,7 +110,7 @@ UI later. Concretely: full untruncated `detail`, a copy button, no
 "something went wrong, try reloading."
 
 This follows from **power-user-first**, one of the standing principles:
-swarm-ui instances are mostly self-hosted and operators are techies, so
+swarm-ui instances are typically self-hosted and operators are techies, so
 errors should give them what they need to actually diagnose a problem
 rather than a friendly wall. Power-user-first doesn't mean
 newcomer-hostile — the UI should still be self-explanatory, warn or ask