fix: preserve total_additions for inherited prompts and prevent amend inflation

[svarlamov]

Summary

Fixes two related bugs in total_additions/total_deletions handling:

• Bug: total_additions reset to 0 for inherited prompts without checkpoint data #1080: Inherited prompts from INITIAL (without new checkpoint data) had their total_additions zeroed out by calculate_and_update_prompt_metrics using unwrap_or(0)
• git commit --amend inflates total_ai_additions due to cumulative totals being re-summed in merge helpers #1098: During git commit --amend, merge helpers summed cumulative total_additions from both checkpoint VA and blame VA, causing inflation that doubled with each successive amend

Root Cause

#1080: calculate_and_update_prompt_metrics unconditionally overwrote total_additions from the session_additions map. When a prompt was inherited from INITIAL with no new checkpoints, the session_id wasn't in the map, so unwrap_or(0) zeroed out the inherited value.

#1098: merge_prompts_picking_newest and merge_attributions_favoring_first summed total_additions across sources. Since total_additions is cumulative, when the same prompt appeared in both checkpoint VA (with inherited or delta values) and blame VA (with cumulative values), the sum inflated the total.

Fix

1. Conditional update in calculate_and_update_prompt_metrics: Only overwrite total_additions/total_deletions when checkpoint data exists (if let Some)
2. Cumulative session deltas: After metrics calculation, add_inherited_totals adds the inherited INITIAL base to session deltas, making all prompts carry cumulative values
3. Max-based merge: Changed merge_prompts_picking_newest and merge_attributions_favoring_first from saturating_add to max, since the highest cumulative value is always the most current

Closes #1080
Closes #1098

Test plan

• test_inherited_prompt_preserves_total_additions_when_no_checkpoint_data — inherited prompts preserve values, prompts with checkpoints get updated
• test_empty_session_maps_preserve_existing_totals — empty maps (merge/rebase pattern) preserve values
• test_merge_prompts_does_not_inflate_totals_for_same_prompt — same prompt in both sources doesn't double
• test_merge_prompts_picks_higher_cumulative_total — higher cumulative value wins in merge
• test_session_delta_plus_inherited_base_is_cumulative — session delta + inherited base = correct cumulative
• Updated integration test test_merge_prompts_picking_newest_uses_max_totals_on_collision to expect max semantics
• Full test suite passes (cargo test — all green)

🤖 Generated with Claude Code

[devin-ai-integration]

✅ Devin Review: No Issues Found

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