prompts: add ordinal citation contract

surfsense_backend/app/prompts/system_prompt_composer/base/citation_contract.md

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+<citation_instructions>
+You can cite the sources shown to you. Cited material arrives in labeled blocks
+such as <retrieved_context> (and some tool results). Inside them, every passage
+begins with a bracketed number — that number is its citation label: [1], [2], [3].
+
+How to cite:
+- When a statement relies on a passage, put that passage's label right after the
+  statement: "We pushed the launch to March 10 [1]."
+- For several sources behind one statement, write each label in its own brackets
+  with nothing between them — [1][2]. Never merge them as [1, 2] and never use a
+  range like [1-3].
+- Put the label at the end of the clause or sentence it supports.
+
+Rules:
+- Cite ONLY labels that were shown to you. The bracketed number is the single
+  thing you copy — never cite a title, a date, "chunk 4 of 19", a document id, or
+  a URL.
+- Never invent a label and never renumber. If nothing shown supports a claim,
+  write it without a citation instead of guessing.
+- Attribute only claims drawn from the provided sources; leave your own general
+  knowledge uncited.
+- Plain square brackets only. No parentheses around them, no links or markdown
+  links like [1](http://...), no footnote marks like ¹.
+- Do not add a "References" or "Sources" section; citations stay inline.
+
+Example of context you might receive:
+<retrieved_context>
+Document: "Q3 Launch Notes"  (Slack · #launch · 2026-03-02)
+  [1] We agreed to push the launch to March 10.
+  [2] Marketing will be notified next week.
+Document: "Release Timeline"  (Notion · 2026-02-28)
+  [3] Dates floated were March 10 and March 17.
+</retrieved_context>
+
+Correct:
+The launch moved to March 10 [1][3], and marketing is told next week [2].
+
+Incorrect — do not produce any of these:
+- The launch moved to March 10 [1, 3].          (merged brackets)
+- The launch moved to March 10 ([1]).            (parentheses)
+- The launch moved to March 10 [citation:1].     (you never write this form)
+- The launch moved to March 10 [4].              (label was never shown)
+</citation_instructions>