RFAI-04: Typed ProposalCompiler and revision-backed edit-before-approve flow

[Chris0Jeky]

Context

Week 4 from taskdeck-12-week-roadmap-v4.md.

Parent: #972
Depends on: #975

This issue replaces string-first proposal planning for Capture and Chat with a shared typed compiler path while preserving fallback behavior.

Scope

• Add a typed candidate-to-ProposalCompiler path for Capture and Chat.
• Keep the existing grammar planner as a fallback while the typed path stabilizes.
• Add operation-level validation, risk reasons, and structured unsupported-operation failures.
• Add ProposalRevision rows for edit-before-approve. Edits must never destructively overwrite the original proposal payload.
• Upgrade the review card enough to edit title, description, board, column, label, and date before approval.
• Ensure approval executes the latest revision.
• Move invariant tests from advisory/experimental to gating once the compiler path is in place.

Acceptance Criteria

• Capture and Chat share the same compiler path for proposal-producing operations.
• Planner/compiler failure rate is measurable via content-free metrics.
• Users can edit key proposal fields before approval.
• Approval applies the latest saved revision, not stale original state.
• proposal_outcomes distinguishes approved, edited_then_approved, rejected, and ignored.
• Invariant tests are wired into the required CI path once stable.

Suggested Verification

• Backend tests for compiler validation/fallback/revision execution
• Frontend tests for edit-before-approve review interactions
• E2E capture/chat -> proposal -> edit -> approve -> board path