chore: rename boundary command to agent-firewall (#25889)

Renames the `coder boundary` CLI subcommand to `coder agent-firewall` as
part of the Boundaries → Agent Firewall rebrand.

`coder boundary` is retained as a hidden, deprecated alias that prints a
deprecation notice to stderr before running. Both commands use separate
builder functions backed by the same boundary base command and license
verification logic.

Closes https://linear.app/codercom/issue/AIGOV-236

<details><summary>Implementation notes</summary>

**Approach:** Two separate `*serpent.Command` objects (not `Aliases`) so
the deprecated `boundary` path can print a stderr warning while
`agent-firewall` stays clean.

**Changes:**
- `enterprise/cli/boundary.go`: Split old `boundary()` into
`buildAgentFirewallCmd()` and `buildBoundaryAliasCmd()`. Error messages
in `verifyLicense` now reference "agent-firewall".
- `enterprise/cli/root.go`: Register both commands.
- `cli/root.go`: Update YAML-only option validation bypass for the new
command name.
- Tests: Rename to `TestAgentFirewallSubcommand`, add
`TestBoundaryAlias`, update license verification tests to use
`agent-firewall`.
- Golden files and CLI reference docs regenerated.
- `docs/ai-coder/agent-firewall/version.md` and `docs/manifest.json`
updated.

</details>

> Generated with [Coder Agents](https://coder.com/agents) by @SasSwart

enterprise/cli/testdata/coder_--help.golden

@@ -14,9 +14,9 @@ USAGE:
        $ coder templates init
 
 SUBCOMMANDS:
-    aibridge               Manage AI Bridge.
-    boundary               Network isolation tool for monitoring and restricting
+    agent-firewall         Network isolation tool for monitoring and restricting
                            HTTP/HTTPS requests
+    aibridge               Manage AI Bridge.
     external-workspaces    Create or manage external workspaces
     features               List Enterprise features
     groups                 Manage groups

enterprise/cli/testdata/coder_agent-firewall_--help.golden

@@ -1,7 +1,7 @@
 coder v0.0.0-devel
 
 USAGE:
-  coder boundary [flags] [args...]
+  coder agent-firewall [flags] [args...]
 
   Network isolation tool for monitoring and restricting HTTP/HTTPS requests