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/tools/hivectl-cli.md

@@ -643,7 +643,7 @@ Stream a snapshot to the swarm's snapshot store over the WireGuard mesh — the
 
 Nothing is staged locally: `btrfs send` writes straight into the connection, so a multi-gigabyte agent needs no scratch space on this host. The mesh is the authentication (cryptokey routing binds the sender's address to its key), so there is no credential to pass here.
 
-There is no destination argument: a swarm has one store, read from `services.hyperhive.swarm.snapshotStore`.
+No destination argument exists: a swarm has one store, read from `services.hyperhive.swarm.snapshotStore`.
 
 **Usage:** `hivectl agent subvol snapshot push [OPTIONS] <LABEL>`