compact: retained_turns を retained_tokens に置換

保護単位をターン数からトークン量に変更。compact 時のカット位置は
Pod::split_for_retained() で UsageRecord を逆算ソースとして決定し、
ターン境界ではなくアイテム単位で切る。デフォルトは 8000 トークン。

crates/manifest/src/config.rs

@@ -86,7 +86,7 @@ pub struct CompactionConfigPartial {
     #[serde(default)]
     pub compact_request_threshold: Option<u64>,
     #[serde(default)]
-    pub compact_retained_turns: Option<usize>,
+    pub compact_retained_tokens: Option<u64>,
     #[serde(default)]
     pub provider: Option<ProviderConfigPartial>,
 }
@@ -241,9 +241,9 @@ impl CompactionConfigPartial {
             compact_request_threshold: upper
                 .compact_request_threshold
                 .or(self.compact_request_threshold),
-            compact_retained_turns: upper
-                .compact_retained_turns
-                .or(self.compact_retained_turns),
+            compact_retained_tokens: upper
+                .compact_retained_tokens
+                .or(self.compact_retained_tokens),
             provider: merge_option(self.provider, upper.provider, ProviderConfigPartial::merge),
         }
     }
@@ -371,9 +371,9 @@ impl TryFrom<PodManifestConfig> for PodManifest {
                         .unwrap_or(defaults::PRUNE_MIN_SAVINGS),
                     compact_threshold: c.compact_threshold,
                     compact_request_threshold: c.compact_request_threshold,
-                    compact_retained_turns: c
-                        .compact_retained_turns
-                        .unwrap_or(defaults::COMPACT_RETAINED_TURNS),
+                    compact_retained_tokens: c
+                        .compact_retained_tokens
+                        .unwrap_or(defaults::COMPACT_RETAINED_TOKENS),
                     provider: comp_provider,
                 })
             })

crates/manifest/src/defaults.rs

@@ -18,9 +18,11 @@ pub const PRUNE_PROTECTED_TURNS: usize = 3;
 /// [`crate::CompactionConfig::prune_min_savings`].
 pub const PRUNE_MIN_SAVINGS: u64 = 4096;
 
-/// Number of most-recent turns retained after a compact. See
-/// [`crate::CompactionConfig::compact_retained_turns`].
-pub const COMPACT_RETAINED_TURNS: usize = 2;
+/// Token budget retained (unchanged) at the tail of the history across
+/// a compact. Items whose cumulative token count fits within this budget
+/// starting from the end are kept verbatim; the rest are summarised.
+/// See [`crate::CompactionConfig::compact_retained_tokens`].
+pub const COMPACT_RETAINED_TOKENS: u64 = 8000;
 
 /// Default instruction asset reference used when `worker.instruction`
 /// is omitted. See the `PromptLoader` prefix addressing scheme for the

crates/manifest/src/lib.rs

@@ -195,9 +195,11 @@ pub struct CompactionConfig {
     #[serde(default)]
     pub compact_request_threshold: Option<u64>,
 
-    /// Number of recent turns retained after compaction.
-    #[serde(default = "default_compact_retained_turns")]
-    pub compact_retained_turns: usize,
+    /// Token budget retained verbatim at the tail of the history after
+    /// compaction. Measured against the occupancy estimate from
+    /// `UsageRecord` history; turn boundaries are ignored.
+    #[serde(default = "default_compact_retained_tokens")]
+    pub compact_retained_tokens: u64,
 
     /// Optional provider for the compactor (summary) LLM.
     /// If omitted, the main provider is cloned via `clone_boxed()`.
@@ -211,8 +213,8 @@ fn default_prune_protected_turns() -> usize {
 fn default_prune_min_savings() -> u64 {
     defaults::PRUNE_MIN_SAVINGS
 }
-fn default_compact_retained_turns() -> usize {
-    defaults::COMPACT_RETAINED_TURNS
+fn default_compact_retained_tokens() -> u64 {
+    defaults::COMPACT_RETAINED_TOKENS
 }
 
 impl Default for CompactionConfig {
@@ -222,7 +224,7 @@ impl Default for CompactionConfig {
             prune_min_savings: default_prune_min_savings(),
             compact_threshold: None,
             compact_request_threshold: None,
-            compact_retained_turns: default_compact_retained_turns(),
+            compact_retained_tokens: default_compact_retained_tokens(),
             provider: None,
         }
     }
@@ -357,7 +359,7 @@ model = "claude-sonnet-4-20250514"
         assert_eq!(c.prune_min_savings, 4096);
         assert_eq!(c.compact_threshold, Some(80000));
         assert_eq!(c.compact_request_threshold, None);
-        assert_eq!(c.compact_retained_turns, 2);
+        assert_eq!(c.compact_retained_tokens, 8000);
     }
 
     #[test]