feat(console): let a new agent be given a display name

[raullopez-sandbox]

Fourth and last piece of the display-name work: the setting itself. The
column landed in #326 and the bridges learned to render it in #330 and
#338 — until now nothing in the product could actually set it.

What changes

The new-agent form gets an optional Display name field beside Name,
plumbed through the gateway client exactly the way icon_url already is:
nullable, presentation-only, never consulted for routing. agents.name
stays the lowercase identifier that rooms, sessions and the command
palette key off; display_name is only ever shown.

Two behaviours worth calling out, both covered by tests:

• Accepting the slug offer keeps what you typed. Type Switch Dev
  into Name, the charset rejects it, you click Use switch-dev — the
  identifier becomes switch-dev and Switch Dev moves into Display
  name. It is a perfectly good human name, just not a valid identifier,
  so it should not be thrown away.
• A display name you wrote yourself is left alone — including one you
  deliberately cleared. Emptying the field is the answer "show this agent
  under its identifier", not an absence to be filled in. This is the
  autoApproveTouched idiom already used elsewhere in the form; a falsy
  check on the value alone passes every other test and fails that one.

Also passes the gateway's own error detail through to the rejection
banner. GatewayError.message is prefixed with the raw status line,
which its own doc comment says "reads as noise in a form" — detail is
the sentence the server actually wrote.

Base

This targets main, not #338. It is Console TypeScript and its real
prerequisites — the display_name column (#326) and the slug-suggestion
form (#325) — are both already merged. It shares no files with the bridge
PRs, so stacking it on them would only couple its CI to theirs.

Verification

Format, lint and typecheck clean. Browser 178/178, node 2999/3000 (the
one failure is a pre-existing /var vs /private/var macOS path test in
session-spawner.test.ts, untouched here and green on Linux CI),
connector-assets 10/10.

The form was also recorded end-to-end driving the real
AgentIdentityFields component and useConfigureAgentForm hook, with
Playwright reading the resulting input values back off the page:
switch-dev / Switch Dev, then switch-dev / Release Captain.

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