[Bug]: Session resumption after context clear can skip approval gates by misreading COMPLETED status

## Description

When a Claude session is cleared mid-workflow (cache expiry, going to lunch, deliberate fresh start) and a new session resumes by reading `aidlc-state.md`, the new instance can incorrectly proceed past a human approval gate if the stage status is `COMPLETED` but approval has not yet occurred.

## Steps to Reproduce

1. Run AIDLC through a generation stage (e.g., Requirements Analysis) until the artifact is produced
2. At this point, `aidlc-state.md` shows `Requirements Analysis | COMPLETED` — the generation step is done, but the user has not yet approved
3. Clear the session context (cache expires, take a break, start a fresh Claude session)
4. Resume by asking the new session for the current status
5. The new session reads `aidlc-state.md`, sees `COMPLETED`, and proceeds to the next stage (e.g., Workflow Planning) without waiting for approval

## Expected Behavior

A resumed session should recognize that `COMPLETED` on a generation stage means the artifact exists but not that approval was given. It should halt at the approval gate and report: "Requirements Analysis artifact has been generated — awaiting your approval before proceeding."

## Actual Behavior

The resumed session treats `COMPLETED` as fully done (including approved) and proceeds to the next stage. In the observed case, `execution-plan.md` was generated before `requirements.md` had been approved. The state file and audit log had to be manually corrected afterward.

From the session's own audit log entry after the error was caught:

> "Resumed session. Incorrectly read aidlc-state.md as showing Requirements Analysis COMPLETED and proceeded to Workflow Planning without verifying against the audit trail. Audit trail correctly showed 'awaiting user approval before proceeding to Workflow Planning' — requirements were never formally approved."

## Root Cause

`COMPLETED` is used to mean two different things:
- The artifact generation step finished
- The user approved the artifact

A fresh-context instance reading the state file has no way to distinguish between these two states from the single `COMPLETED` value.

## Suggested Fix

Introduce distinct status values so generation-complete and approval-complete are unambiguous to any reader with zero prior context:

- `GENERATED` or `AWAITING_APPROVAL` — artifact exists, user has not yet approved
- `COMPLETED` — artifact approved; stage fully closed

Alternatively, make human approval an explicit row in the Stage Progress table so the gate is visible as its own incomplete step rather than an annotation on a `COMPLETED` row.

This matters most for natural context resets (cache TTL, breaks, deliberate fresh starts) — not just formal handoffs. The state file should be written defensively for a zero-context reader at all times.

Start by tracing how a resumed session reads aidlc-state.md and compare that interpretation with the audit log entry. Review the Requirements Analysis, requirements.md, and execution-plan.md examples to map generation completion against approval. Done means a zero-context reader can distinguish an unapproved generated artifact from an approved stage and cannot proceed past the approval gate.

Written by the indexing model from the issue text.