docs: the plan for splitting "what the content is" from "what the machine does"

[wu908]

Answering the creator's three questions — why three values, doesn't that constrain the content, doesn't it constrain what the AI produces — with the conclusion that the enum is not the problem: one field is doing two jobs.

Why the split, not the delete

Machine behaviour needs a comparable finite key: routing, which judgment fields are required, which signals are watched, and — verified, not assumed — which learned rules apply at all:

content_genome.py:147
if query["content_intent"] and applicability["intent"] != query["content_intent"]:
    continue        # this rule does not apply to this piece

So the field is load-bearing beyond cosmetics: before #124 a piece of text was judged solve, which meant experience distilled for share pieces was silently not used. "What is this content" needs to be open, because the creator should never pick from three and the AI should be able to name any shape it sees. Deleting the enum pushes interpretation into 12 files; splitting it keeps the machine's key finite and lets the naming be free.

The plan (additive, staged, reversible, production usable throughout)

1. Add content_form (open, AI-named) as pure plumbing — no visual change at all.
2. Show the AI's name, demote the three values to the behaviour key — this is the step with visible change; the confirm step's copy becomes 「这篇要读者拿走什么」.
3. Rename the behaviour key in the rubric so it stops reading as a content category.
4. Only if a fourth behaviour appears, widen the six CHECK constraints in one migration — explicitly not for a new content shape.

Inventory is measured, not estimated

references	139 across 12 files (content_genome 30, content_opportunity 17, intent_actions 11)
tables carrying the CHECK	six (020×2, 023, 025, 026, 027; deliverables nullable)
user-visible render sites	five
E2E assertions on intent labels	none — so the copy carries less risk than feared

Also records the migration trap that has now bitten twice (any new column on content_projects must be added to the 043 rebuild DDL constant), and the acceptance criteria including one test to keep content_form out of routing.