feat: add phase 3 prompt customization backend

- parse prompt model and argument-hint metadata and persist model on prompt invocations
- expand markdown-linked file context in scanned prompt and instruction bodies
- discover .claude/rules instructions and support Claude-style paths metadata
- discover customization roots up to the nearest parent repository and watch them for changes
- apply per-turn prompt model overrides before sidecar execution

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -57,7 +57,7 @@ Aryx is a desktop app that turns GitHub Copilot into a full workspace. Connect r
 | Mid-turn steering | Send follow-up messages while an agent is running — input is injected immediately |
 | Plan review & questions | Agents propose plans and ask clarifying questions before acting |
 | Run timeline | Structured history of tool calls, delegations, hooks, and context usage |
-| Copilot customization | Auto-discovers repo instructions (`copilot-instructions.md`, `AGENTS.md`, `CLAUDE.md`, `.instructions.md`), agent profiles, and prompt files from your repo, then auto-rescans when those files change |
+| Copilot customization | Auto-discovers repo instructions (`copilot-instructions.md`, `AGENTS.md`, `CLAUDE.md`, `.instructions.md`, `.claude/rules`), agent profiles, and prompt files across nested repo roots, expands Markdown-linked context, exposes prompt metadata (`tools`, `model`, `argument-hint`), then auto-rescans when those files change |
 | Model & effort tuning | Choose models, adjust reasoning effort, and set interaction modes per session |
 
 ### Developer tooling