Track session position

[Soph]

If multiple commits are made by one agent session we are including the session log in each commit as it grows. This means if you only look at one of the commit you would need to look at prior commits to figure out which parts of the session belong to this commit.

To make this easier we track now the position and last uuid of any user message in the log at the beginning of a prompt. This is then written to metadata.json so we can easily figure out which parts of the log actually belong to the session.

Quick visualization:

  TRANSCRIPT FILE (grows over time)
  ═══════════════════════════════════════════════════════════════════════════════

  Line │ UUID    │ Type      │ Content
  ─────┼─────────┼───────────┼──────────────────────────────────────────────────
     0 │ u-001   │ user      │ "Add a login form"
     1 │ a-001   │ assistant │ {usage: {input: 1000, output: 200}} "I'll create..."
     2 │ a-002   │ assistant │ [tool_use: Write login.tsx]
     3 │ u-002   │ user      │ [tool_result: success]
     4 │ a-003   │ assistant │ {usage: {input: 1200, output: 150}} "Done!"
  ─────┴─────────┴───────────┴──────────────────────────────────────────────────
                             ▲
                             │  CHECKPOINT 1 saved here
                             │  metadata.json: {
                             │    "transcript_lines_at_start": 0,
                             │    "transcript_uuid_at_start": ""
                             │  }
  ─────┬─────────┬───────────┬──────────────────────────────────────────────────
     5 │ u-003   │ user      │ "Now add password validation"
     6 │ a-004   │ assistant │ {usage: {input: 2500, output: 300}} "I'll add..."
     7 │ a-005   │ assistant │ [tool_use: Edit login.tsx]
     8 │ u-004   │ user      │ [tool_result: success]
     9 │ a-006   │ assistant │ [tool_use: Task → spawns agent-abc123]
    10 │ u-005   │ user      │ [tool_result: agentId: abc123]
    11 │ a-007   │ assistant │ {usage: {input: 3000, output: 400}} "Validation added!"
  ─────┴─────────┴───────────┴──────────────────────────────────────────────────
                             ▲
                             │  CHECKPOINT 2 saved here
                             │  metadata.json: {
                             │    "transcript_lines_at_start": 5,      ◄── knew to start here
                             │    "transcript_uuid_at_start": "a-003" ◄── last UUID at checkpoint 1
                             │  }
  ─────┬─────────┬───────────┬──────────────────────────────────────────────────
    12 │ u-006   │ user      │ "Add remember me checkbox"
    13 │ a-008   │ assistant │ {usage: {input: 4000, output: 250}} "Adding..."
    14 │ a-009   │ assistant │ [tool_use: Edit login.tsx]
    15 │ u-007   │ user      │ [tool_result: success]
    16 │ a-010   │ assistant │ {usage: {input: 4200, output: 100}} "Done!"
  ─────┴─────────┴───────────┴──────────────────────────────────────────────────
                             ▲
                             │  CHECKPOINT 3 saved here
                             │  metadata.json: {
                             │    "transcript_lines_at_start": 12,
                             │    "transcript_uuid_at_start": "a-007"
                             │  }

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