feat(runtime): engage discourse planner on cold pack/teaching path

Option 2 of the lane-isolation work.  Mirrors the existing warm-path
hook into the cold-start branch (``gate_decision.fire`` ⇒ stub
response): after ``_maybe_pack_grounded_surface`` succeeds and the
result is pack- or teaching-grounded, build the same DiscoursePlan
and replace ``pack_surface`` with the rendered plan whenever it has
more than one move.

This closes the gap option 1 exposed: cold-start one-shot prompts
("Tell me about truth.", "Describe wisdom.", "Give me an example of
truth.") now produce deterministic multi-clause output without any
priming setup — the planner becomes the spine for grounded surfaces,
not a warm-only sidecar.

Gating discipline preserved:
* Engages only when pack_source_tag in {"pack", "teaching"}.  Cases
  routed to vault, none, or the discovery-signal disclosure are
  untouched.
* BRIEF mode collapses to a single ANCHOR move which renders
  byte-equivalent to the existing pack-grounded composer, so
  flag-off cognition byte-identity is preserved.
* Empty bundles → empty plan → no surface change (planner is total).

A/B on multi_sentence_response (21 cases, public/v1):

  flag off: multi=0.1429, primed_multi=0.0000, conn=0.0769
  flag on : multi=0.5238, primed_multi=0.5000, conn=0.2308

Cold-start lift: multi +38pp, conn +15pp.  Primed metric unchanged
(those cases already engaged the warm hook in step 5).

Sample cold-start surfaces flag-on:
* "Tell me about truth."
  → "Truth is a claim or state grounded by evidence and coherent
    judgment. Furthermore, truth belongs to cognition.truth. In
    turn, truth grounds knowledge."
* "Describe wisdom."
  → "Wisdom is sound judgment informed by knowledge and experience.
    Furthermore, wisdom belongs to cognition.wisdom. In turn,
    wisdom orders judgment."
* "Give me an example of truth."
  → "Truth is a claim or state grounded by evidence and coherent
    judgment. In turn, truth grounds knowledge."  (EXAMPLE mode:
    anchor + relation, no support)

Gates all green:
* flag off: cognition eval byte-identical
  - public 100/100/91.7/100, holdout 100/100/83.3/100
* smoke suite 67/67
* conversational_thread_coherence: 3 unwanted placeholders flag off
  and flag on (no regression)
* 136/136 planner + grounding + intent + lane tests pass

chat/runtime.py

@@ -879,6 +879,41 @@ class ChatRuntime:
                 pack_source_tag = "none"
             else:
                 pack_surface, pack_source_tag = pack_result
+                # Option 2 — engage discourse planner on the cold
+                # pack/teaching-grounded path so one-shot prompts (no
+                # priming) can produce multi-clause output when the
+                # planner has substrate.  Same gating discipline as
+                # the warm hook: only fires when grounding source is
+                # pack or teaching, and only replaces the surface
+                # when the plan has more than one move.  BRIEF mode
+                # collapses to a single ANCHOR move and renders to a
+                # surface byte-equivalent to the existing composer,
+                # so the flag-off path is unaffected.
+                if (
+                    self.config.discourse_planner
+                    and pack_source_tag in {"pack", "teaching"}
+                ):
+                    from generate.discourse_planner import (
+                        plan_discourse,
+                        render_plan,
+                    )
+                    from generate.grounding_accessors import (
+                        grounding_bundle_for,
+                    )
+                    from generate.intent import classify_response_mode
+                    from generate.intent_bridge import (
+                        classify_intent_from_input,
+                    )
+
+                    _cintent = classify_intent_from_input(text)
+                    _cmode = classify_response_mode(text)
+                    if _cintent.subject:
+                        _cbundle = grounding_bundle_for(_cintent.subject)
+                        _cplan = plan_discourse(_cintent, _cmode, _cbundle)
+                        if len(_cplan.moves) > 1:
+                            _crendered = render_plan(_cplan)
+                            if _crendered:
+                                pack_surface = _crendered
             self._context.finalize_turn(
                 empty_result,
                 tokens_in=tuple(filtered),