swarm-matrix-ctl: one control binary for the matrix container, not one per job

Renames `swarm-matrix-minter` and reshapes it around subcommands. Minting
is now `swarm-matrix-ctl mint`.

Running rust inside `containers.hive-matrix` is not free: it needs its own
store identity, its own cert role and its own bind mounts, and every one of
those is per-*container*, not per-task. A second single-purpose crate would
have had to duplicate that plumbing to add one action, so the next thing
that has to run in there should be a verb here rather than a new crate.
The old name guaranteed the opposite.

`main.rs` is clap dispatch; the minting logic moves to `mint.rs` unchanged.
A bare invocation is refused: `mint` writes a credential, so "no verb"
defaulting to it would make a typo in the unit mint rather than fail.

The environment prefix moves with it, `MATRIX_MINTER_*` → `MATRIX_MINT_*`.
Scoped to the verb and not to the binary, because a binary-scoped prefix is
one the next verb has to share or widen, and a widened one never narrows
again. A test asserts every variable carries the verb's prefix.

The principal renames too. The cert role, bao policy, granting unit, leaf
filename and `certAuthCns` entry all have to spell one string the same way,
so leaving them as `swarm-matrix-minter` would have rebuilt the naming
split this branch exists to remove. Renaming the nix options alongside is
free here: every one of them is introduced by this PR and has never been
released, so no operator config names them yet.

`ExecStart` now names the verb, which is a contract between a nix string
and a clap enum that fails at deploy time with no local signal. Both ends
assert it: `mint_is_spelled_the_way_the_unit_invokes_it` in the crate, and
a new module-eval arm reading the rendered `ExecStart`.

docs/getting-started/setup.md drops the sender token from its "live on the
host" list: setup does not touch this credential, so a setup guide has no
reason to name it.

docs/getting-started/setup.md

@@ -293,7 +293,7 @@ See [`tools/hivectl.md`](../tools/hivectl.md) for every `hivectl` verb.
 
 - **No forge admin token is stored in any agent state dir.** Agents
   hold a regular agent token in their `forge-token` file; sensitive
-  creds (the core token, the matrix sender token) live on the host.
+  creds (the core token) live on the host.
 - All config changes (forge PRs on `agent-configs/<name>`) go through
   operator approval — agents can't unilaterally rebuild containers, by design.
   See [`boundary.md`](../trust-boundary/boundary.md) and [`security.md`](../trust-boundary/security.md).