Adds internal/symlinks package that encodes in code the convention
previously maintained by hand on the VM: every Claude account home
must expose `session-env`, `file-history` and `projects` as symlinks
to a single shared target, so account failover does not create
divergent state (duplicate JSONL transcripts, broken undo history).
- EnsureForAccount(home, required) creates missing links and target
directories, refuses to auto-correct a divergent link (risks data
loss), and errors when a regular file sits where the link belongs.
- ValidateAll(homes, required) aggregates errors across both accounts
so the operator sees every problem at once rather than fixing one
per restart cycle.
- RequiredShared exposes the production defaults so lifecycle and
switcher (A2/A3) can depend on it directly.
9/9 unit tests green.
Part of Phase 1 Chantier A — Failover robuste.
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