[Feat] Export a notebook, answer or note with its citations intact

[mrsibe]

Context

There is no export path in KnowNote today: an answer, its citations and a note can be read in the app, and copied out — losing the citation links, which is the thing that makes the answer worth keeping.

The stronger version of the same request recurs for NotebookLM: Markdown/LaTeX export, full export, and "keep the citation cross-references with the export". Export is where the value of the provenance work either leaves the app or does not.

Tracked for v1.6, not v1.5: v1.5 (#154) is deliberately narrowed to Trust & Sources inside the app. Export is how that trust travels. This is not blocked by the v1.5 work, but it is better after exact-passage citation lands so the exported references carry spans.

Proposal

Export an answer, a note, or a whole notebook as Markdown, with citations preserved as anchors back to the source and page.

## Answer

Attention is O(n²) in sequence length [1].

[1]: attention.pdf, p.7 — "the self-attention layer connects all positions …"

• Citation markers become numbered references; each reference carries source title, page and the supporting passage.
• Optionally emit a source-anchored form: knownote://source/<documentId>?page=7&start=… so a receiving KnowNote opens the exact location; the plain Markdown form links to the page when a URL exists.
• Notes export their excerpts with the same source-anchor metadata — reuse buildExcerptMarkdown(), do not re-invent it.
• PDF export is out of scope for the first version; Markdown is the format researchers re-use.
• No export may drop a citation silently: an unresolvable citation is emitted with its last-known location, not removed.

Acceptance

• A single answer exports to Markdown with all resolved citations as references
• A note exports with its excerpt anchors intact
• A full notebook exports answers + notes + source list
• An unresolvable citation is exported with its snapshot, never dropped
• Round-trip: an exported source anchor re-opens the right page and span in KnowNote

Verification

Export a notebook with one answer citing two sources and one excerpt-bearing note; re-import the Markdown into a scratch note and click each anchor.

Depends on

#69 / #70 (citations), #73 (excerpt anchors), exact-passage citation (#155).