Add amika send and amika sessions commands

[jdc123]

What

Adds the CLI half of the agent-sessions feature: a one-shot amika send and a sessions browser, driving the new remote agent-sessions API.

• amika send [message] — send a message to a coding agent. With neither --session-id nor --sandbox, a sandbox is created behind the scenes and a new chat starts; the returned session_id continues it next time. Agent from --agent (claude|codex), else the org default, else claude. Message from a positional arg or stdin. Synchronous — prints the reply. Flags: --agent, --session-id, --sandbox, --new-session, --repo. In text mode session_id/created-sandbox notes go to stderr so stdout stays the pure response; --output json emits the AgentSessionSendResponse verbatim.
• amika sessions list / amika sessions show <id> — browse the durable chats those sends create (backed by GET /agent-sessions).
• API client: SendAgentSession, ListAgentSessions, GetAgentSession + mirror types. SendAgentSession uses the same 10-minute timeout as the existing synchronous agent-send.

Server dependency

Requires the agent-sessions endpoint in amika-mono (gofixpoint/amika-mono#866). The client structs are hand-maintained mirrors of that API's OpenAPI schema, matching how the rest of apiclient works.

Testing

• httptest coverage of the three client methods: request method/path/body, full response parsing, nullable sandbox_id/agent, []-not-null list, and id path-escaping.
• Command tests: send/sessions registration + flags, message-required, and --output validation.
• make fmtcheck vet lint and go test ./internal/apiclient/... ./cmd/amika/... pass; the CLI builds (make build-cli).

Streaming (update)

amika send now streams the reply in real time via the new
POST /agent-sessions/stream SSE endpoint (amika-mono#915), instead of
blocking on the whole message.

• API client: SendAgentSessionStream(req, handlers) POSTs with
  Accept: text/event-stream, forwards status (sandbox lifecycle) and
  delta (agent text) frames to callbacks as they arrive, and returns the
  terminal done frame — the same AgentSessionSendResponse the buffered
  SendAgentSession returns. A mid-stream error frame, or a stream that
  ends without done, becomes an error; auth/validation failures arrive as a
  JSON error before the stream opens and are surfaced like the buffered path.
• amika send --stream: streams by default when stdout is a terminal;
  buffers when stdout is piped (so a captured pipe stays the single final
  response, not the concatenated deltas) and always for --output json (which
  must stay one valid object). --stream / --stream=false forces either
  mode. Sandbox progress + session_id go to stderr; only agent text goes to
  stdout, matching the buffered path. Buffered SendAgentSession is unchanged.

The --session-id returned by either mode continues the chat as before.

Testing (streaming)

• httptest SSE coverage of SendAgentSessionStream: in-order status/delta
  dispatch + done parsing (and Accept/method/path), an error frame → error,
  a stream with no done → error, and a pre-stream 4xx → error.
• send command test now also asserts the --stream flag is registered.

Note: the streaming code was added in an environment without a Go toolchain,
so it was hand-verified (tab-clean, imports checked) rather than compiled
locally — CI (make fmtcheck vet lint + go test) is the gate.

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