Add BCE temporal tags to all documents; add temporal-dating steering doc

.kiro/steering/temporal-dating.md

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+---
+inclusion: always
+---
+
+# Temporal Dating Conventions for Ancient History
+
+## BCE Tag Syntax
+
+Factbase supports BCE dates in temporal tags. Always use them — never leave BCE dates untagged.
+
+Supported formats:
+- `@t[=331 BCE]` — exact event (Battle of Gaugamela)
+- `@t[~280 BCE]` — approximate/state (Pharos Lighthouse built)
+- `@t[305 BCE..30 BCE]` — date range (Ptolemaic Egypt)
+- `@t[..612 BCE]` — ended at date (fall of Nineveh)
+- `@t[911 BCE..]` — started, ongoing from that point
+
+Use `~` prefix for approximate dates common in ancient history. Most pre-classical dates are approximate.
+
+## Dates as Source Validation
+
+In ancient history, dates ARE the scholarship. A date like "~2334 BCE" for Sargon of Akkad encodes decades of archaeological and textual debate. Temporal tags serve double duty:
+
+1. **Factbase temporal tracking** — enabling `as_of` and `during` queries
+2. **Source accountability** — every tagged date must have a footnote citing who established that chronology
+
+When answering temporal review questions, always:
+- Add the `@t[...]` tag with BCE syntax
+- Confirm the source footnote covers the dating (not just the event)
+- Note if the date is contested (e.g., Egyptian chronology has high/middle/low variants)
+
+## Ranges vs Exact Dates
+
+- Reigns, periods, dynasties → use ranges: `@t[336 BCE..323 BCE]`
+- Battles, founding events → use exact: `@t[=331 BCE]`
+- Approximate dates → use tilde: `@t[~2560 BCE]`
+- Unknown dates → use `@t[?]` and note why in the review answer