fix: link Claude subagent tool calls (#459)

## Summary
- Link Claude `Task` / `Agent` tool calls to child subagent sessions
using result-side `toolUseResult.agentId` when
  queue/progress mapping is absent.
- Preserve sibling Claude subagent tool calls when Claude emits additive
same-`message.id` assistant chunks.
- Keep streaming snapshot behavior by replacing progressive partial text
with the latest text block while still
  preserving distinct additive content blocks.

  ## Context
This is the Claude-side version of the inline subagent-linking contract
addressed for Codex in #444 / #458.

The UI already renders inline subagent sessions when
`tool_calls.subagent_session_id` is populated. This PR makes
Claude ingestion populate that existing field in more cases, so child
sessions that are already ingested can appear
inline under the exact parent `Task` / `Agent` tool call that launched
them.

  ## Backfill note
This changes parsed tool-call metadata for existing Claude rows.
Existing archives will need a full parser
  reprocess / data-version bump to backfill inline links.

This PR intentionally does not bump `dataVersion`; maintainers should
decide whether to trigger that reparse.

  ## Validation
  - `go test ./internal/parser ./internal/sync -count=1`
  - `git diff --check origin/main...HEAD`

  If you add screenshots later, I’d add a final section:

  ## Visual Check   
- Before: Claude rendered the three sibling `Task` calls, but none had
an inline linked subagent session. <img width="960" height="463"
alt="image"
src="https://github.com/user-attachments/assets/f0b0417e-476f-4763-ba43-dc49e469e4c9"
/>
- After: The same three sibling `Task` calls are preserved. <img
width="959" height="514" alt="image"
src="https://github.com/user-attachments/assets/f5c15e57-6737-41a0-8e33-71d3c610d10a"
/>

---------

Co-authored-by: Wes McKinney <wesmckinn+git@gmail.com>

Author: wbaik

internal/db/db.go

@@ -27,10 +27,19 @@ import (
 // trigger a non-destructive re-sync (mtime reset + skip cache
 // clear) so existing session data is preserved.
 //
-// Bumped to 25: Codex parser now also links codex_app subagents
+// Bumped to 26: Claude parser now (a) links Task / Agent tool
+// calls to child subagent sessions via toolUseResult.agentId
+// when queue/progress mappings are absent, populating
+// tool_calls.subagent_session_id, and (b) merges additive
+// same-message.id assistant chunks instead of keeping only the
+// last entry, preserving sibling tool_use blocks and
+// progressively-built text. Existing rows need re-parsing so
+// these linkages and merged content show up.
+//
+// (25: Codex parser now also links codex_app subagents
 // via collab_agent_spawn_end event_msgs, wait_agent function
 // calls, and agent_path subagent notifications. Existing rows
-// need re-parsing so codex_app subagent linkage works.
+// need re-parsing so codex_app subagent linkage works.)
 //
 // (24: Codex parser now annotates spawn_agent tool calls
 // with subagent_session_id once the spawned agent id is known.
@@ -68,7 +77,7 @@ import (
 //
 // (17: Codex <skill> template filtering.)
 // (16: <turn_aborted> system messages.)
-const dataVersion = 25
+const dataVersion = 26
 
 const tokenCoverageRepairStatsKey = "token_coverage_repair_v1"
 

internal/db/messages.go

@@ -436,6 +436,29 @@ func (db *DB) MaxOrdinal(sessionID string) int {
 	return int(n.Int64)
 }
 
+// LastClaudeMessageID returns the claude_message_id of the
+// highest-ordinal assistant message in a session whose
+// claude_message_id is non-empty, or "" if none exists. The sync
+// engine uses this to detect cross-sync splits of a single
+// streaming response (next sync's first appended assistant entry
+// shares the message.id of the previously-stored last assistant).
+func (db *DB) LastClaudeMessageID(sessionID string) string {
+	var s sql.NullString
+	err := db.getReader().QueryRow(
+		`SELECT claude_message_id FROM messages
+		 WHERE session_id = ?
+		   AND role = 'assistant'
+		   AND claude_message_id != ''
+		 ORDER BY ordinal DESC
+		 LIMIT 1`,
+		sessionID,
+	).Scan(&s)
+	if err != nil || !s.Valid {
+		return ""
+	}
+	return s.String
+}
+
 // savedPin captures the minimal pin state needed to re-attach a pin
 // after a full message replacement. source_uuid is the preferred
 // identifier because it survives rewrites where the ordinal stream