feat(cloud-agent-next): discover and send remote slash commands

[eshurakov]

Summary

Adds end-to-end remote slash command support so Cloud Agent sessions
running against a live CLI wrapper can discover the wrapper's command
catalog and submit structured /command messages instead of only free
text. This mirrors the existing list_models discovery flow but for
the list_commands relay command.

• CLI live transport (cli-live-transport.ts): discovers commands
  via list_commands, parses with the new
  remoteCommandCatalogV1Schema, and publishes commands.available
  events. Adds per-owner in-flight dedupe and generation guards so a
  catalog from a replaced/stale owner is ignored, the catalog is
  retained across transient refresh failures, and is cleared on owner
  disconnect, malformed payloads, or oversized responses. Legacy CLIs
  that reject list_commands get an empty catalog. send now forwards
  command payloads to send_command with protocolVersion,
  command, arguments, optional messageID, and model/variant.
• Schema (schemas.ts): new remoteCommandCatalogV1Schema with
  bounds (max 256 commands, 2 KB strings, 32 hints, 512 KB serialized)
  and a transform that drops source: 'skill' commands from the relay
  catalog.
• Session manager (session-manager.ts): ignores commands.available
  emitted by a previous session after a switch, via the existing switch
  generation guard.
• Web hook (useSlashCommandSets.ts + new
  slash-command-selection.ts): replaces the unconditional pinned-default
  fallback with session-type-aware selection — Cloud Agent sessions still
  fall back to the pinned default catalog when the wrapper list is empty,
  remote sessions use only the exact CLI catalog, and read-only/unresolved
  sessions expose nothing.
• Mobile composer (chat-composer.tsx + new
  chat-composer-slash-commands.ts, slash-command-suggestions.tsx):
  surfaces autocomplete suggestions while typing /, inserts the selected
  command into the input, and routes structured command submissions
  through a new onSendCommand prop. session-detail-content.tsx wires
  this to manager.send({ payload: { type: 'command', ... } }).
• Relay DO (UserConnectionDO.ts): treats list_commands the same
  as list_models for pending-request dedupe (CATALOG_REQUEST_PENDING)
  and the 512 KiB response size cap (CATALOG_TOO_LARGE), via a shared
  isCatalogCommand helper.

Architecturally, the command discovery path reuses the model discovery
pattern (owner-scoped in-flight tracking + generation invalidation) rather
than introducing a new coordination mechanism. The mobile composer splits
slash-command parsing into a pure helper (chat-composer-slash-commands.ts)
so the composer component stays focused on rendering.

Verification

No manual verification was performed in this session — the work consisted
of committing and pushing the implementation; no local runtime or in-app
testing was done.

• [ ]

Visual Changes

Before	After
Mobile composer had no slash command autocomplete	Mobile composer shows /command suggestions above the input and inserts the selected command
(add before screenshot)	(add after screenshot)

Reviewer Notes

• Companion PR (CLI side): Cli - Support remote slash command discovery and execution kilocode#12169
  implements list_commands/send_command handling in the CLI wrapper.
  This PR is the cloud-side counterpart and should be reviewed together
  with it.
• The discoverCommands flow closely mirrors discoverModels; the risk
  area is the generation/owner guards ensuring a stale or in-flight
  catalog doesn't leak across owner changes or reconnects — covered by
  the new transport tests (replaced owner, in-flight dedupe, transient
  failure retention, malformed/oversized clearing).
• remoteCommandCatalogV1Schema silently filters source: 'skill'
  commands; confirm that's the intended policy for the relay catalog.
• Mobile onSendCommand is a new required prop on ChatComposer;
  verify all call sites pass it.