fix(facts): truncate Fact::new timestamps to microseconds

Linux's Utc::now() returns nanosecond precision; our LanceDB schema
stores Timestamp(Microsecond, UTC). On Linux CI the nanosecond tail
was dropped at storage time, so equality checks after roundtrip
failed (facts_roundtrip_through_record_batch, get_finds_inserted_fact).

macOS's Utc::now() was already microsecond-truncated in practice, so
local tests didn't catch this.

Fix: Fact::new() now truncates `created_at` via
chrono::SubsecRound::trunc_subsecs(6). In-memory timestamps now
always match what the schema can store, so roundtrip is lossless.

Any caller that sets `created_at` / `occurred_at` directly with
nanosecond precision is still on the hook — but Fact::new covers
the common path and the extract/collect pipelines use
from_timestamp-style constructors that are microsecond-based.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: linggen

src/facts/types.rs

@@ -9,7 +9,7 @@
 //! Schema matches `doc/tech-spec.md`. Any field change here must also update
 //! `schema.rs` and bump the store's format version.
 
-use chrono::{DateTime, Utc};
+use chrono::{DateTime, SubsecRound, Utc};
 use serde::{Deserialize, Serialize};
 use std::fmt;
 use std::str::FromStr;
@@ -80,7 +80,11 @@ pub struct Fact {
 
 impl Fact {
     /// Minimal constructor for a fresh fact. Generates a new id and stamps
-    /// `created_at` to `now`. All optional fields default to `None` / empty.
+    /// `created_at` to `now`, truncated to microsecond precision so the
+    /// value round-trips through LanceDB (which stores `Timestamp(Microsecond)`).
+    /// Without the truncation, Linux platforms where `Utc::now()` returns
+    /// nanosecond precision would fail equality checks after store roundtrip.
+    /// All optional fields default to `None` / empty.
     pub fn new(content: impl Into<String>, r#type: FactType, origin: Origin) -> Self {
         Self {
             id: Uuid::new_v4(),
@@ -92,7 +96,7 @@ impl Fact {
             outcome: None,
             origin,
             cwd: None,
-            created_at: Utc::now(),
+            created_at: Utc::now().trunc_subsecs(6),
             occurred_at: None,
             source_session: None,
         }