eval: guard shared workspace render ownership

[schickling-assistant]

Paired with compoundingtech/st2#95 and compoundingtech/st2#97.

Discriminator

Adds one model-free cell for two active declarations that share a workspace:

• different effective bytes for .st2/PERSONA.md must fail validation
• full materialization must fail before either owner writes
• targeted materialization must not bypass the unselected sibling's claim
• diagnostics must name the target and both owners
• byte-identical shared claims remain valid and idempotent

The cell starts no PTY, Claude, or Codex session. It also updates the generated
catalog, explicit model-free harness exclusion, and eval VRS mapping.

AGENT-SPEC.md remains pinned to its current accepted st2 commit in this PR;
the new behavior should become normative there only when the paired runtime PR
lands and the pin advances.

Proof

With st2 PR #97 commit d691d71:

SCORE: 7 PASS / 0 FAIL / 7 gating judges
VERDICT: PASS

Additional free gates:

• 73 KDL files parse
• 46 maintained eval folders load; 9 nested catalogs validate strictly
• model-free exclusion inventory matches exactly
• no-PII check clean
• generated CATALOG.md check clean
• git diff --check clean

The complete harness-contract gate is currently blocked by the ambient
commit-msg identity hook rejecting existing synthetic fixture authors
(seed@local) while materializing the pre-existing signal-rename cells; this
cell is not implicated.

Posted on behalf of @schickling

field	value
agent_name	co2-bear
agent_session_id	a078daee-6f98-4916-91a8-d21291407789
agent_tool	Codex CLI
agent_tool_version	0.145.0
agent_runtime	Codex CLI 0.145.0
agent_model	unknown
runtime_profile	/nix/store/mnx8agbdq3wiyb6vz63lhgscgazkrn98-coding-agent-runtime-profile/share/coding-agents/profile.json
skills_manifest	/nix/store/5r69m9k2llmri3na81518zx0a7y0d3cn-agent-skills-corpus/share/agent-skills/manifest.json
worktree	evals/schickling-assistant/2026-07-30-shared-workspace-render-eval
machine	dev3
tooling_profile	dotfiles@0fb7e03