feat(token-usage): attribute subagent usage to the subagent's own session

Usage is now leaf-attributed for every tool: subagent and fork
transcripts own their usage under their own session, carrying the
parent as the parent_session_id/external_parent_session_id attributes
— the same identity model SessionEvents already use, so TokenUsage and
SessionEvents finally agree about which session a transcript is. The
parent-inclusive total becomes a derived rollup over the parent
relationship instead of the stored identity, keeping per-agent cost
visible; recording everything under the parent was lossy in the other
direction.

Previously the worker rolled every stream with an
external_parent_session_id into its parent. For Codex forks that
contradicted both the spec ("sessions are per rollout file") and
ccusage, hiding fork children entirely; for Claude sidechains it
matched ccusage but differed from git-ai's own SessionEvents. This is
now a single standardized rule, with the Claude difference from
ccusage documented (per-session groupings differ; aggregate totals
match). Cross-file dedup of sidechain replays is unaffected — entry
dedup is global, not session-scoped.

tracked_files persists the parent id so DB-only reconciliation carries
the same relationship attributes; schema v5 is a pre-release reset
(existing rows are keyed by the rolled-up parent and don't record
their source rollout, so they cannot be reassigned in place). The
server already ingests both parent attributes into
token_usage_v2_events columns; no wire change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>