…pad MCP call — closes D-kr2-st3

Mechanical follow-on swap unlocked by K-8 (substrate `bd165eb2`) +
KR-7a's MCP client transport. Same pattern as KR-7 chain-emit:
write_scratchpad_entry body replaces `raise
ScratchpadWriteNotAvailableError()` with
`await mcp_client.invoke('kora__write_agent_scratchpad', {...})`,
returning the substrate-assigned scratchpad_entry_id. ~40 LOC across
scratchpad.py + provider.py + tools/iso_node.py + tests + docs.

Production-test posture per IsoKron PM #27:
* K-8's kora__write_agent_scratchpad handler is a
  notImplementedHandler stub on substrate main; dispatch tier (task
  NousResearch#395) un-stubs + bridges Layer-A wsk_* auth → Layer-B actor_kind.
* KR-8 code shape is sound; mock tests verify correctness.
* Production deploys wait on dispatch tier landing — identical
  posture to KR-7 / KR-7b / KR-7a.

K-DG note: K-8's internal flow emits the substrate-canonical chain
literal `kronicle.agent_scratchpad.created` (NOT
`kora.scratchpad.entry.created` as some prior specs may suggest).
The runtime doesn't pass an event_type — the substrate emits
internally as part of the SECDEF flow. Verify-at-first-live-emit:
confirm event_log.actor_id resolves to the 0076-seeded canonical
Kora actor.

scratchpad.py:
* write_scratchpad_entry body: live call to mcp_client.invoke. K-8
  contract output: {'scratchpad_entry_id': '<uuid>',
  'approved_event_id': '<uuid>'} — projected to a plain str
  scratchpad_entry_id return.
* BLAKE3 content_hash computed via existing
  compute_scratchpad_content_hash() helper; passed to MCP tool.
* Defensive: None mcp_client raises ValueError; unexpected response
  shape raises RuntimeError (same pattern as KR-7's emit_kora_event).
* mcp_client param now required (was Optional with deferred-error
  default). Forward signature stays caller-stable from KR-2 ST3.
* ScratchpadWriteNotAvailableError class kept exported tagged
  [kora.isokron.deprecated] for one release. Original deferred-tag
  preserved inside the message for grep stability.

provider.py:_attempt_scratchpad_write:
* Stop catching ScratchpadWriteNotAvailableError (no longer raised).
* Fetch IsoKronMCPClient via self._connection.get_mcp_client();
  failure logs ERROR [kora.scratchpad.write.failed] + returns.
* Catch IsoKronMCPInvocationError + defensive Exception at the
  lifecycle boundary — log at ERROR so failures are operator-visible
  (same posture as KR-7's chain emit). Sessions don't crash; writes
  drop with a loud log line.
* Successful writes log INFO [kora.scratchpad.write] with the
  substrate-assigned entry_id and invalidate the per-workspace
  own-scratchpad cache so the next read re-fetches.

tools/iso_node.py:
* _handle_iso_node_create + _handle_iso_node_supersede: dropped
  the deferred-envelope return path. On success return
  {"ok": True, "entry_id": <substrate-uuid>}. On
  IsoKronMCPInvocationError return
  {"ok": False, "substrate_error": True, "tool_name": ...,
   "message": ...} — structured signal for the model that maps to
  substrate-level failures distinct from validation errors.
* iso_node_supersede also passes the new entry_id into the
  kora.node.superseded chain event payload so audit consumers can
  trace the supersession chain.
* Removed unused ScratchpadWriteNotAvailableError import.

Tests (5 new + 9 updated):
* test_scratchpad.py:
  - Replaced test_write_scratchpad_entry_raises_deferred_write_error
    with 4 MCP-call-path tests (happy/error/None-client/bad-shape)
    + 1 deprecation-runway test.
  - _FakeProviderConnection now exposes get_mcp_client() returning a
    _FakeMcpClient; sync_turn/on_memory_write tests flipped from
    deferred-WARNING assertions to success-INFO + entry_id checks.
  - New test_sync_turn_substrate_error_logs_at_error_session_continues
    verifies the IsoKronMCPInvocationError → ERROR-log path.

* test_iso_node_tools.py:
  - _FakeProviderConnection gains get_mcp_client(); new _FakeMcpClient
    routes by tool_name (kora__write_agent_scratchpad → spe-mock-N;
    kora__append_event → evt-mock-N; kora__read_kora_capability_row
    → minimal matrix).
  - test_iso_node_create_returns_deferred_payload →
    test_iso_node_create_returns_ok_envelope_with_substrate_entry_id;
    new test_iso_node_create_substrate_error_surfaces_structured_envelope.
  - test_iso_node_supersede_inherits_node_kind_from_original flipped
    to assert success + verify the inherited node_kind in the new
    content_inline header + verify the kora.node.superseded emit
    payload carries the new entry_id.

* test_tool_finalize.py: round-trip tests flipped from deferred to
  success-envelope assertions. _FakeProviderConnection gains
  get_mcp_client().

* test_provider_end_to_end.py: _FakeMcpClient extended to handle
  kora__write_agent_scratchpad (returns spe-mock-N). E2E assertions
  flipped from 3 deferred-WARNINGs to 3 [kora.scratchpad.write] INFO
  logs + 3 scratchpad_write_calls on the fake + 2 append_event_calls.
  Assertion narrowed to NOT count the cap-matrix fetch (KR-7b adds
  that; this PR stays narrow per parallel-mergeability).

BUILD_DEVIATIONS:
* D-kr2-st3-no-scratchpad-write-mcp-tool moved Open → Closed with
  Rule-5 spec-quote, KR-8's call-site refactor inventory, production-
  test posture, deprecation-runway note, verify-at-first-live-emit
  step.
* 1 deferral remains open (D-kr3-st2 — KR-9 dispatchable).

README "Operator pitfalls":
* Deferred-surface table trimmed 2 → 1 row (D-kr3-st2 only).
* D-kr2-st3 moves to Recently-closed list.
* Individual "Scratchpad writes are currently deferred" pitfall
  rewritten as "Scratchpad writes route via kora__write_agent_
  scratchpad (KR-8)" with success/failure log paths + operator
  grep pointer.

Local gates:
* ty check — 7,337 diagnostics, zero-delta vs KR-7 baseline.
* pytest tests/plugins/memory/ — 357/357 passing.
* Full suite via xdist (-n auto): 24,654 / 168 failed / 129 skipped.
  Δ vs KR-7b (24,652/175/129): +2 passed, -7 failed. Same
  tests/tools/* + tests/tui_gateway/* xdist isolation noise; none
  touch isokron.

1 deferral still open pending CC#1 substrate:
* D-kr3-st2-no-relationlink-write-mcp-tool ← KR-9 (K-10 merged; PM dispatched)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>