The architectural unlock for Kora's memory. Ships the structural
skeleton of plugins/memory/isokron/ — a MemoryProvider subclass that
will replace Hermes' flat MEMORY.md/USER.md once ST2-ST4 fill in
reads + writes against the IsoKron substrate.

Architecture decision (PM lean accepted, no Rule-3 ASK):
* Reads → asyncpg pool, direct Postgres on IsoKron substrate
* Writes → MCP client to Sea MCP server (authorization + audit)
* No direct DB writes anywhere in this provider — structural
  invariant enforced by raising NotImplementedError on every write
  path until ST3 wires the MCP client.

Files added:
* plugins/memory/isokron/__init__.py — register(ctx) + env-var
  expansion + config loader from plugins.entries.isokron
* plugins/memory/isokron/provider.py — IsoKronMemoryProvider class.
  Required ABC methods implemented (name, is_available, initialize,
  get_tool_schemas). All 12 optional hooks override the ABC defaults
  to raise NotImplementedError with a "[kora.isokron.todo]" Rule-6
  message so operators can grep their logs.
* plugins/memory/isokron/connection.py — IsoKronConnection wrapper.
  Dedicated-thread asyncio loop (matches tools/mcp_tool.py upstream
  pattern). pg_pool() and mcp_client() accessors raise until ST2/ST3.
* plugins/memory/isokron/config.py — pydantic v2 IsoKronProviderConfig
  with field-level validation: postgres:// DSN scheme check, MCP
  endpoint stdio/http(s) transport check, 0 <= cache_ttl_seconds
  <= 3600, extra="forbid" for typo detection. Plus
  ISOKRON_CONFIG_SCHEMA for kora memory setup.
* plugins/memory/isokron/plugin.yaml — manifest with pip deps + hooks
* plugins/memory/isokron/README.md — architecture diagram (hybrid
  reads/writes), config example, KR-2 ST1-ST4 surface table,
  operator pitfalls
* tests/plugins/memory/test_isokron_provider_skeleton.py — 29 tests:
  plugin discovery + register, construct + lifecycle, config
  validation (5 negative tests + env-var expansion), 12 stub-method
  Rule-6 message verifications, static metadata, connection
  accessor NotImplementedError checks.

Files modified:
* pyproject.toml — new [project.optional-dependencies].isokron
  extra: asyncpg==0.31.0, mcp==1.26.0, blake3==1.0.7 (blake3 needed
  for ST3 scratchpad content_hash verification).
* uv.lock — regenerated for the new extra.

Test layout adapted (NOT a Rule-3 deviation): bucket said
`plugins/memory/isokron/__tests__/test_provider_skeleton.py`, repo
convention is `tests/plugins/memory/test_<name>_provider.py`. I
followed the repo convention because pytest auto-discovers there;
the bucket's `__tests__/` path is JavaScript convention, not Python.

Architecture STOP-gate cleared (per ST1 spec): the MCP client
surface in upstream Hermes (tools/mcp_tool.py) exposes a clean
Python client. No Rule-3 ASK needed; ST3 will wire the MCP write
path on top.

Verification:
* 29 ST1 smokes pass serially in 0.80s.
* `from plugins.memory import discover_memory_providers` finds
  isokron at position 5 of 9 with the manifest description.
* ty check: 7,341 diagnostics — identical to ST1/ST2/ST3/ST4
  baseline. Zero new diagnostics from KR-2 ST1.
* Full suite (xdist parallel) — running in background, will be
  appended to PR description.

Rule-6 honest labels: ST1 ships the skeleton, NOT a working
provider. `memory.provider: isokron` in config.yaml will half-work
today — provider registers + initializes, but every call to
prefetch/sync_turn/system_prompt_block raises NotImplementedError.
Operators should keep the Hermes-inherited default (flat MEMORY.md)
until ST2 lands.

Bucket: KR-2 sub-task 1 / 4. Architecture decision documented in
plugins/memory/isokron/README.md §"Architecture (hybrid, PM-leaned)".

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