BR-5: Replace Serena fully (memory -> SIN-Brain, symbol-tools -> Simone)

[Delqhi]

Plan: docs/plans/serena-memory-migration.md

Serena has TWO roles, both better placed elsewhere:

1. Memory (.serena/memories/*.md, passive) -> SIN-Brain (active, self-editing)
2. Symbol/code tools (find_symbol, replace_symbol_body, ...) -> Simone-MCP (already in stack)

After migration: no passive parallel memory, no duplicate symbol-tool layer. Serena removed; Simone stays as WRITE layer.

Acceptance:

• Importer idempotent + dry-run; imported items carry provenance=serena-seed
• No code path writes to .serena/memories/ anymore
• No Serena symbol-tools in configs/hooks; edits go through Simone
• 1:1 mapping Serena-tool -> Simone-tool documented
• Legacy files archived (not deleted in-place)

Non-goal: Simone-MCP is NOT touched/replaced (it takes over Serena's edit role).
Depends on: SIN-Brain SB-1, SB-6; relates BR-6.