feat(ui): add plugin command registry

docs/core-data-structures/commands.md

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+# Human Commands
+
+The human-command seam of [`dsh-commands`](../../packages/ui/commands). TUI and ACP adapters use it to discover and directly execute plugin-owned commands for an exact agent without creating a model message. The [command RFC](../rfc/implemented/feature/2026-07-19-plugin-command-registration.md) owns dispatch and lifecycle rationale; the [package README](../../packages/ui/commands/README.md) owns composition and limitations.
+
+Source: [`packages/ui/commands/src/index.ts`](../../packages/ui/commands/src/index.ts)
+
+## Surface and input metadata
+
+A definition selects one or more adapter identities. The shipped identities are `tui` and `acp`; the string intersection keeps the registry extensible without widening editor autocomplete to plain `string`. ACP currently exposes one unstructured-input hint.
+
+```ts type-equiv
+/** A UI adapter capable of listing and executing human commands. */
+type CommandSurface = 'tui' | 'acp' | (string & {})
+```
+
+```ts type-equiv
+/** Immutable command input metadata compatible with ACP unstructured input. */
+interface CommandInputDescriptor {
+  /** Placeholder shown before the user supplies free-form input. */
+  readonly hint: string
+}
+```
+
+## Definition
+
+`CommandDefinition` is the plugin-authored registration. Omitted surfaces resolve to both shipped adapters; the registry validates and freezes a detached effective definition.
+
+```ts type-equiv
+/** Plugin-owned command registration. */
+interface CommandDefinition {
+  /** Lowercase command name without the leading slash. */
+  readonly name: string
+  /** Human-readable summary used in discovery UI. */
+  readonly description: string
+  /** Optional free-form input hint advertised to capable clients. */
+  readonly input?: CommandInputDescriptor
+  /** Surfaces exposing this command; omission means both shipped surfaces. */
+  readonly surfaces?: readonly CommandSurface[]
+  /** Execute against the receiving agent without sending the command to the model. */
+  readonly handler: (invocation: CommandInvocation) => CommandResult | Promise<CommandResult>
+}
+```
+
+## Invocation and result
+
+The adapter owns cancellation and passes the exact target agent. `rawInput` begins immediately after the parsed name and retains the adapter-delivered separator and suffix. Results are direct UI outcomes, not tool results or session events.
+
+```ts type-equiv
+/** Invocation passed to one registered command handler. */
+interface CommandInvocation {
+  /** Exact agent whose human-facing surface received the command. */
+  readonly agent: Agent
+  /** UI adapter that dispatched the command. */
+  readonly surface: CommandSurface
+  /** Exact text following the registered command name, including separator whitespace. */
+  readonly rawInput: string
+  /** Cancellation signal owned by the dispatching UI request. */
+  readonly signal: AbortSignal
+}
+```
+
+```ts type-equiv
+/** Expected command outcome rendered directly by the dispatching UI. */
+type CommandResult =
+  | { readonly kind: 'success'; readonly text?: string }
+  | { readonly kind: 'error'; readonly text: string }
+```
+
+## Discovery and parsing views
+
+Adapters receive handler-free immutable descriptors after scope resolution and surface filtering. `parseCommand()` returns `ParsedCommand` before registry resolution; syntax-valid input can still name an unavailable command.
+
+```ts type-equiv
+/** Handler-free immutable command view returned to UI adapters. */
+interface CommandDescriptor {
+  /** Lowercase command name without the leading slash. */
+  readonly name: string
+  /** Human-readable summary used in discovery UI. */
+  readonly description: string
+  /** Optional free-form input hint advertised to capable clients. */
+  readonly input?: CommandInputDescriptor
+  /** Surfaces on which this definition is visible. */
+  readonly surfaces: readonly CommandSurface[]
+}
+```
+
+```ts type-equiv
+/** Syntactically valid slash command before registry resolution. */
+interface ParsedCommand {
+  /** Lowercase command name without the leading slash. */
+  readonly name: string
+  /** Exact text following the command name. */
+  readonly rawInput: string
+}
+```