Replace the standalone `hermes portal topup` subcommand with an in-session
/credits slash command — a focused money surface (balance in, top-up out) that
works in the CLI, TUI, and every messaging platform from one registry entry.

- commands.py: register /credits (Info category). Slack is at its 50-slash cap,
  so /credits is routed via /hermes credits on Slack only (new
  _SLACK_VIA_HERMES_ONLY set) to avoid clamping a canonical command off the
  native list and breaking Telegram parity; native everywhere else.
- account_usage.py: build_credits_view() — one portal fetch → balance lines +
  identity line + org-pinned top-up URL + depleted flag, consumed by all
  surfaces. Reuses the same snapshot/URL builder as /usage so numbers match.
- cli.py: _show_credits() — balance block + identity line + 3-button panel
  (Open top-up / Copy link / Cancel) via the existing prompt_toolkit modal.
  ASK, never auto-launch; headless falls back to printing the URL.
- gateway/slash_commands.py: _handle_credits_command() — renders the block +
  tappable top-up URL + no-wait copy; works on button and plain-text platforms.
- /usage credits line now points to /credits.
- Retire `hermes portal topup` (portal_cli.py back to baseline); the engine
  (slug/name parse + nous_portal_topup_url) stays as the shared core.

No polling, no payment confirmation (billing phase 2a). Depends on NAS #409.

In the TUI, /credits runs in the slash-worker subprocess where there is no
live prompt_toolkit app and stdin is the JSON-RPC pipe. _show_credits called
the 3-button modal unconditionally, which fell back to reading stdin →
exception → slash.exec rejected → the command produced no output (only the
pre-existing 'Credit access paused' banner showed).

- _show_credits: when self._app is None (TUI worker / piped / non-interactive),
  render the text variant — balance block + tappable top-up URL + no-wait line,
  same affordance as the messaging surfaces — and skip the modal entirely. The
  3-button panel still renders in the interactive CLI.
- Depleted banner copy: 'run /usage for balance' → 'run /credits to top up'
  now that /credits is the dedicated money surface (+ tests).
- Regression tests: _show_credits with self._app=None renders text and never
  invokes the modal; logged-out path.

Add a credits.view JSON-RPC method returning the structured CreditsView
(logged_in, balance_lines, identity_line, topup_url, depleted) so the TUI can
render a clickable <Link> top-up button instead of plain text. Account-
independent (portal fetch gated on a logged-in Nous account), fail-open to
{logged_in: false} on any hiccup. Mirrors session.usage's credits-block pattern.

Frontend (TUI-local /credits command + Ink component) lands separately.

TUI-local /credits: fetches the structured balance via the credits.view RPC,
prints the balance + identity + top-up URL, then arms the EXISTING confirm
overlay (Enter = open top-up in browser via openExternalUrl, Esc = cancel).
Reuses ConfirmReq — no new overlay component/state/input handler. Headless
(openExternalUrl returns false) falls back to printing the URL.

- gatewayTypes.ts: CreditsViewResponse.
- commands/credits.ts: the command (mirrors /status's rpc+guarded pattern).
- registry.ts: register creditsCommands.
- test: balance+overlay armed, headless fallback, no-url, logged-out (4 cases).

Matches the CLI /credits 'Enter to open' affordance. Phase 2a: no polling.