chore(update-stack): rework skill as 2-phase ISO merge, remove stack-maintainer agent

[PierreBrisorgueil]

Problem

The update-stack skill was a flat list of steps without a clear mental model. The agent was redundant with CLAUDE.md guardrails and never used organically.

Expected behaviour

Two explicit phases:

1. ISO merge — stack modules and lib come out identical to upstream, explicit --theirs rules per file category
2. Project alignment — project modules aligned via MIGRATION.md + tasks reference + ERRORS.md patterns

Agent removed — its responsibilities are covered by CLAUDE.md always-on guardrails.