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/agent-lifecycle/agent-hierarchy.md

@@ -178,8 +178,8 @@ nspawn agent. Open questions, not yet wired:
 
 One harness serve binary (`hive-agent`, with its `hive-agent-mcp`
 sibling), one shared `nix/agent-modules/` tree, one service unit
-(`systemd.services.hive-agent`) for all agents. There is no separate
-manager service name or role distinction in the harness — privilege
+(`systemd.services.hive-agent`) for all agents. No separate manager
+service name or role distinction exists in the harness — privilege
 differences live server-side in the broker socket (which tool groups
 and manager-surface calls each agent receives).