Convert all IPC client commands to output structured JSON:
- list: JSON array of sessions instead of text table
- eval: structured JSON with stdout/stderr/value/error fields
- send: JSON {accepted: bool}
- shutdown: JSON {accepted: bool}
- session/history: already JSON (no change)

Remove the `status` subcommand (superseded by `session` which
returns a superset of the same information via IPC).

Add structured error handling:
- Errors output as JSON to stderr with code/message/hint fields
- Distinct exit codes by error category (2=transport, 3=session,
  4=protocol)
- R evaluation errors are returned as normal JSON responses
  (exit 0) to distinguish from IPC failures

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Copilot review:
- Distinguish protocol errors (malformed JSON-RPC response) from
  transport errors (connection refused, timeout) in send_request(),
  using appropriate exit codes and error codes
- Fix eval output format docs: value/error are omitted when not
  applicable (not present as null), clarify silent mode behavior

Agent review (bd task 31tr):
1. cmd_list: use serde_json::to_value(SessionInfo) instead of manual
   JSON construction to prevent field sync issues
2. cmd_* return types: change from Result<()> to () since all error
   paths use exit_error (never return Err)
3. exit_error testability: add doc comment explaining the design
   trade-off (process::exit prevents in-process testing; covered by
   integration tests)
4. Deduplicate expect messages: extract format_json() helper, use
   distinct message prefix
5. Remove misleading "exit code 1 reserved" comment
6. EMPTY_RESPONSE: clarify message indicates possible server bug
7. Test JSON parse errors: add parse_ipc_json() helper that includes
   stdout/stderr in panic message for debugging

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- Use e.chain().any() to find serde_json::Error through anyhow's
  context wrapper, replacing fragile string matching
- Fix format_json expect message to match function name

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

When an IPC request is rejected because the user is typing, include
the current editor buffer content in the error message. This lets
AI agents see what the user is typing and decide whether to retry
or abort.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- Move editor buffer from error message to JSON-RPC error.data field
  ({"buffer": "..."}) to keep message stable for pattern matching
- Add IpcResponse::error() and error_with_data() constructors, use
  throughout to avoid struct literal repetition
- Thread error.data through server to JsonRpcResponse
- Replace cmd_list expect with exit_error for structured error output
- Add integration tests for exit codes and structured JSON errors:
  - SESSION_NOT_FOUND (exit 3) with JSON stderr
  - list always returns valid JSON (even with no sessions)
  - Protocol error / timeout (exit 4) with JSON stderr

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- Cap editor buffer in USER_IS_TYPING error data at 1024 characters to
  prevent excessively large payloads. Include buffer_truncated and
  buffer_original_length fields for completeness.
- Clarify docs/CHANGELOG distinction between transport-level timeouts
  (exit 2, socket/pipe failures) and server-side request timeouts from
  --timeout (exit 4, JSON-RPC error).
- Document USER_IS_TYPING data fields in ipc.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- Fix module doc comment to include data field in error format
- Add history_session_id to list help text and docs example
- Use runtime-derived PID in session_not_found test to avoid flakiness

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>