[P2] Session-log template: add failed-approaches section and chain links

[jsirish]

From docs/AUDIT-v0.4.0.md (P2, item 7). Competitor-inspired (REMvisual/claude-handoff).

Failed approaches are the most expensive thing for a future session to rediscover, and the current Phase 3 template in skills/throughline-handoff/SKILL.md has no home for them - they end up (or get dropped) in "What happened".

Proposal:

1. Add a "What we tried (including what failed)" section to the session-log template, with guidance to record real evidence (commands, numbers, error text) rather than summaries.
2. Add a chain link: each session log names its predecessor (previous log for the same work stream), so a reader can walk the decision history without the durable HANDOFF.md having to carry it.