Step 2 of the discourse-planner sequencing: add the presentation-depth
axis ResponseMode (brief / explain / walkthrough / paragraph / example)
as a sibling to IntentTag in generate/intent.py, with a deterministic
rule-based classify_response_mode classifier next to classify_intent.
ResponseMode previously lived in generate/discourse_planner.py; moved
to generate/intent.py so the dependency is one-way (planner imports
from intent, never reverse). discourse_planner.py now re-exports.
Additive-only invariant preserved:
* DialogueIntent fields unchanged (tag/subject/secondary_subject/
relation/frame). No equality breakage anywhere downstream.
* classify_intent branches untouched.
* Callers compose (classify_intent(t), classify_response_mode(t))
rather than threading mode through DialogueIntent.
41 new tests pin: placement (canonical home + re-export identity),
classifier behavior (parametrized over 25 prompts), priority ordering
(paragraph > explain, walkthrough > explain), purity (no clock/env/
filesystem), classify_intent invariance (definition / narrative /
example / cause / verification representative cases), and orthogonality
(intent and mode compose, neither shadows the other).
Verification:
* 96/96 existing intent tests pass.
* 69/69 new contract + characterization + classifier tests pass.
* smoke suite 67/67.
* cognition eval byte-identical: public 100/100/91.7/100,
holdout 100/100/83.3/100.
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