Make chat session creation use one canonical path

[chubes4]

Problem

ChatOrchestrator creates sessions through the create-session ability when available, then falls back to direct conversation-store writes when the ability is unavailable.

Code reference on origin/main (008878b6):

• inc/Api/Chat/ChatOrchestrator.php:680 — // Fallback: direct store access.

Why this matters

This is a dual path for a core chat operation. The fallback can hide ability registration/bootstrap failures and makes it harder to reason about the canonical session creation contract.

Acceptance criteria

• Decide the canonical internal path for creating chat sessions.
• If the ability is canonical, remove the direct-store fallback and fail loudly when the ability is unavailable.
• If the store is canonical, make the ability a thin wrapper over the same implementation and have ChatOrchestrator call that shared service directly.
• Tests cover session creation failure when dependencies are unavailable rather than silently bypassing the canonical path.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: 1.0 technical-debt audit and issue drafting; Chris requested tracking issues for the findings.