Closes W-013 wiring debt. Per Phase 2 operator decision: wire
core.cognition.explain into the live core chat REPL.
Changes:
- core/cognition/explain.py: add explain_from_intent(intent, correction_text)
companion to explain() — same dispatch table, skips the full
CognitiveTurnResult round-trip. Callers with only a DialogueIntent can
use this directly.
- chat/runtime.py: add _last_intent and _last_input_text instance fields;
store intent on every classify_intent_from_input() call (pack-grounded
path and stub/empty-vault path); add explain_last_turn() -> str method
that calls explain_from_intent(_last_intent, correction_text=_last_input_text).
- core/cli.py: in cmd_chat REPL loop, handle "/explain" command — calls
runtime.explain_last_turn() and prints the canonical prompt restatement
(or a "no prior turn" message to stderr if no turn has run yet).
- tests/test_explain_repl.py: 11 tests pinning explain_from_intent dispatch
for all intent tags and the ChatRuntime.explain_last_turn() contract.
Per ADR-0017 (Responsive-with-Axiology): introspection is per-turn and
operator-invoked, never autonomous — the /explain command is correct
placement for this feature.
