U4: Build agent intake guidance and parser adapter

[breadback00-dev]

What is the work

Build the format-aware intake lane that converts structured input or messy pasted text into editable source items for the Events Roundup format.

Planned areas:

• lib/intake/
• convex/intake.ts
• docs/agent-guidance/events-roundup-intake.md
• tests/fixtures/intake/
• lib/intake/*.test.ts
• convex/intake.test.ts

Why does it matter

This is the automation lane that removes repetitive Canva-era content loading. It must make messy local event information structured and reviewable without silently inventing facts.

What does done mean

• Structured event-like input becomes source item candidates.
• Messy pasted text becomes candidate source items with review notes.
• Ambiguous or missing facts are flagged, not guessed silently.
• Intake runs are persisted separately from approved source items.
• Agent guidance exists as product-owned documentation/config, not hidden ad hoc prompts.

Acceptance criteria

• Structured input creates candidate event items with expected fields.
• Messy pasted event text creates candidate event items with review notes.
• Ambiguous dates, prices, venues, and source claims are flagged.
• Empty or unrelated paste produces a safe no-approved-items result.
• Intake output requires editor review before slide creation.

Blockers

• Blocked by U2: Define core Convex domain model #2.
• Blocked by U3: Seed product-managed Events Roundup format #3.

What can make this fail

• The intake lane outputs final slides instead of reviewable source items.
• The parser is not format-aware and starts guessing generic fields.
• Agent guidance is buried in implementation code with no reusable editorial rules.
• Tests depend on a live AI provider instead of deterministic fixtures/mocks.

How should Codex verify it

• Run intake unit tests with structured and messy fixtures.
• Run Convex intake tests for persisted intake runs and review states.
• Include fixtures for ambiguity, missing fields, and unrelated paste.
• Confirm no unreviewed intake result can become an approved slide path.