refactor(core): split note content planning by state

[phernandez]

Phase 1 of #1205 (Constructive Domain Modeling tracker).

Makes the note-content planner's contract visible to the type checker instead of enforcing
it at runtime:

• plan_note_content_reconciliation(current: NoteContentState | None, ...) is split into
  bootstrap_note_content_plan(*, observed) -> NoteContentBootstrap (the only Bootstrap
  construction) and plan_existing_note_content_reconciliation(*, current, observed) -> ExistingNoteContentReconciliationPlan (a closed union excluding Bootstrap).
• The two runtime guards in NoteContentReconciler.reconcile ("missing note_content must
  bootstrap" / "existing note_content cannot bootstrap") are deleted — the split signatures
  prove both properties statically, and after the existing Deferred check the plan type
  narrows into NoteContentUpdatePlan for apply_note_content_update_plan with no
  isinstance needed. No test asserted either guard.
• Behavior-preserving: the concurrent-insert IntegrityError path and all result mapping are
  untouched. Two named functions rather than @overload, matching house style and the
  existing narrowed-alias pattern (NoteContentUpdatePlan,
  NoteMaterializationPublishUpdate).

Verification: planner/reconciler/batch suites 31 passed; just typecheck (ty, rules
all=error) and just lint clean.

Phase 1 checklist on #1205 can be ticked when this merges.

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https://claude.ai/code/session_01CPdSXDbYyhyZ1TwgFnpEv8