feat(tracking): add parent session ID for subagent aggregation

[karl82]

Summary

• Adds parent_session_id TEXT DEFAULT '' column + index to the tracking DB (migration-safe)
• RTK's PreToolUse rewrite hook injects --parent-session=<id> into the rewritten command when CLAUDE_CODE_PARENT_SESSION_ID or RTK_PARENT_SESSION_ID is set — positioned before the subcommand so Clap consumes it without leaking into the underlying command's args
• Global --parent-session CLI flag on Cli writes into a OnceLock override (highest priority), falling back to env vars
• New get_by_root_session() query aggregates parent + all child sessions into one row
• rtk gain --parent-session [<id>] shows the root-session view with per-tree savings

How it works

Parent session (CLAUDE_CODE_SESSION_ID=abc):
  rtk git status              →  session_id=abc, parent_session_id=""

Subagent (CLAUDE_CODE_SESSION_ID=xyz, CLAUDE_CODE_PARENT_SESSION_ID=abc):
  rewrite: git status → rtk --parent-session=abc git status
  rtk git status              →  session_id=xyz, parent_session_id=abc

rtk gain --parent-session
  Root abc  •  2 sessions  •  15 commands  •  12.3k saved  •  82.1%

Notes on --parent-session flag isolation

--parent-session=X is injected before the subcommand (rtk --parent-session=X git status). Clap parses it at the Cli level via global = true; it never appears in the args passed to git/cargo/etc. original_cmd and token counts are unaffected.

Stacked on

PR #3 (feat/gain-session) — per-session savings view.

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