#20 merge hardening: base-aware branch diff + collision assistance (bitter-lesson guardrails)

Umbrella for the merge-flow debt; coordinate with #15/#17 (likely one agent, one PR in cmd/merge.rs). Goals: (a) branch merges compute the diff against a base that preserves already-landed forge state (base-aware), (b) fork-drift surfaces as a precise conflict list (extend SEC-05 file listing to .forge deletions) instead of silent loss, (c) record links to the real PR (see #15). EXPLICIT NON-GOALS so the philosophy holds: no locks, no claim protocols, no auto-resolvers, no merge engine, no CI framework — the CLI only lists what conflicted and stops; the reconciling agent decides. Acceptance: merging fork-drifted branches never silently loses state; conflicts = one line + file list; non-goals enforced in review.