Share proj across Serena agents

[xormania]

What changed

• keep the Git-ignored proj/ planning tree on Serena's shared writable /workspace bind
• retain masking for Git metadata, local environment files, secrets, runtime state, protected rails, and Serena cache state
• fail closed before startup when proj/ is not a real directory or contains sensitive path names, hardlinked files, symlinks, FIFOs, Unix sockets, or other special objects
• document the cooperating-host-writer boundary and the client restart required to replace a stale mount plan
• add static, adversarial, and live Docker coverage for shared read/write behavior and containment regressions

Why

Serena's generated mount override treated proj/ as runtime state and replaced it with an empty read-only directory. Agents using the same checkout therefore could not see or update the user's shared planning and audit documents.

Impact

New Serena sessions see the same proj/ files as host-side agents and can write through to the checkout. Existing Serena sessions must reload or restart; activation alone cannot recreate container mounts.

The practical direct-bind design trusts cooperating concurrent host-side writers to preserve the ordinary-file contract. A hostile concurrent host writer remains outside the container-containment guarantee and would require brokered storage.

Validation

• test-first RED for the old proj/ mask and missing activation guidance
• adversarial RED/GREEN cases for masked-state and protected-rail hardlink aliases, sensitive names, nested symlinks, FIFOs, Unix sockets, and a symlinked proj/ root
• bash tests/serena/config-cases.sh
• bash tests/docker/serena-runtime.sh (Codex, Claude Code, and Grok; live host-visible write-through)
• ./scripts/dev/lint-scripts
• bash tests/agent/policy-verify.sh
• ./tools/validate.sh
• ./tools/containment-lint.sh
• ./scripts/dev/check default quick
• Serena symbol retrieval and file diagnostics for the changed Bash code