fix(bookstack): prevent thinking-text flicker, remove page ID citations

[amrit110]

Problems

1. Thinking-text flicker — Intermediate reasoning text generated by the model before a tool call was streamed to the UI, then wiped by text_clear, causing a jarring flash that users couldn't read before it disappeared.

2. Page ID citations — Custom on-prem models cited pages as "From 'X' (page 61):" using the internal numeric page ID, which is meaningless to users. The model was picking up "id": 61 from the get_page tool response and treating it as a page number. The model was also generating preamble text ("Now I have all the information…") before the answer despite the existing prompt instruction.

Fixes

bookstack_agent/ui/app/components/chat-page.tsx — RAF-based text buffering

text_chunk events are now accumulated in a ref and flushed to React state once per animation frame (requestAnimationFrame). If text_clear arrives before the RAF fires (which happens when thinking-text and the subsequent tool-use event arrive in the same TCP packet / event-loop tick), the buffer is discarded silently — no DOM update, no flicker. Once the gate opens (first RAF fires without a text_clear), subsequent chunks stream in normally per frame.

src/aieng_bot/bookstack/tools.py — strip IDs from get_page response

Removed id, book_id, chapter_id, and updated_at from the get_page tool output. The model now only sees name, markdown, and url — exactly what it needs to answer and cite. With no numeric ID visible, it can't accidentally use one as a "page number."

src/aieng_bot/bookstack/prompts.py — stronger citation and no-preamble rules

• Added explicit ban on preamble phrases ("Now I have all the information…", "Let me synthesize…", etc.)
• Hardened the ## Sources instruction: "NEVER include page numbers, numeric IDs, or any internal identifiers. Use only the page title and its URL." — written to be unambiguous for models that don't follow nuanced instructions as reliably as Claude.

Test plan

• All 48 Python tests pass (pytest tests/bookstack/)
• Ask a multi-step question; verify tool steps appear cleanly with no text flash before them
• Verify the final answer streams in progressively
• Verify ## Sources lists titles + URLs with no (page N) annotations