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/process/conventions.md

@@ -53,7 +53,7 @@ peer's name when the caller knows it's hive-local.
 
 ## Identity = socket
 
-There are no auth tokens on the per-agent unix sockets. The socket
+No auth tokens exist on the per-agent unix sockets. The socket
 *path* identifies the principal; perms come from "who has the
 bind-mount." A sub-agent only sees its own `/run/hive/mcp.sock`;
 hive-c0re owns the host admin socket.
@@ -490,7 +490,7 @@ check derivations they don't:
 
 - **`hivectl-docs`** regenerates `docs/tools/hivectl-cli.md` from
   hivectl's clap tree and **fails if the committed copy is stale**.
-  So **after any change to a hivectl verb or flag, regenerate it**:
+  **After any change to a hivectl verb or flag, regenerate it**:
   ```sh
   nix develop -c cargo run --bin hivectl -- markdown-docs > docs/tools/hivectl-cli.md
   ```