fix(portal): run VS Code BYOAI prompts in agent mode so web fetch works

[KB-x]

What's New

Two improvements to the Open in VS Code BYOAI assistants:

1. They can now load the JVD corpus in VS Code — Design mode returns cited, grounded answers instead of falling back to a limited, ungrounded mode.
2. Prompts self-update — each session, the assistant refreshes itself from the published copy, so future prompt changes reach users without a manual reinstall (with a safe fallback to the copy on hand).

Why

• The earlier change granted a fetch tool but pinned the prompt to ask mode. VS Code only attaches tools in agent mode — the ask agent ignores a tools list — so fetch was inert and the assistant reported "no web access."
• VS Code installs the prompt to disk once, so a change to the prompt would otherwise strand everyone who installed the old copy until they manually reinstalled.

Details

• portal/scripts/generate-snips.mjs — synthesized VS Code prompts use agent: agent with tools: ['fetch'] as the sole whitelisted tool. The explicit list replaces the default agent toolset, so the assistant gets only read-only web fetch — no file-editing, no terminal — preserving the hardened posture.
• 17 SYSTEM_PROMPT.md bundle sources (the generated -byoai-prompt.txt is compiled from these via regenerate-bundle.sh) — added a short, non-blocking self-update step: at the start, fetch the current published version and adopt it if reachable; if the fetch fails, silently continue with the inline copy (worst case = today's behavior). Shared across all platforms — a no-op for the launch-fetch clients (GPT/Claude), a real refresh for VS Code's on-disk installs and any stale pasted copy.
• portal/public/byoai/* — regenerated (mirrored .txt + synthesized .prompt.md).
• portal/public/USING-BYOAI.md — security wording updated to describe the single read-only fetch capability.
• portal/src/data/snips.json — regeneration timestamp.

Testing

• Regenerated: 638 snips / 17 JVDs / 0 warnings.
• bun run build — clean.
• Verified: agent: agent + tools: ['fetch'] frontmatter, and the self-update block present in all 17 .txt and all 17 .prompt.md.

Notes

• Follows up fix(portal): give VS Code BYOAI prompts read-only web access #179 (added the tool but under ask mode).
• One-time reinstall still needed to pick up this change (it's the change that introduces self-update). After that, future prompt-body changes propagate automatically.
• Only the frontmatter/permissions remain frozen on disk — that changes almost never.