Fail closed when agents render conflicting content into a shared workspace target

[schickling-assistant]

Problem

Two active agent declarations may name the same workspace and render different
content to the same workspace-relative destination. st2 currently
materializes each declaration independently in discovery order, so both
materializations succeed and the last writer silently wins.

This makes the effective workspace overlay depend on reconcile order. A
subsequent targeted materialization can flip the file again. Per-agent files
such as a persona/system-instruction overlay are especially hazardous because
one agent can boot from another agent's content.

Minimal reproduction

Create two local agent declarations with:

• the same workspace
• copy "_templates/worker.md" ".st2/PERSONA.md" in one
• copy "_templates/orchestrator.md" ".st2/PERSONA.md" in the other
• different template bytes

Then run:

st2 up --catalog "$CATALOG" --host h --materialize-only

Actual

The command succeeds. Both operations are reported as materialized and
.st2/PERSONA.md contains whichever declaration ran last.

Expected

Workspace-relative render destinations have one coherent desired state.
Before the first workspace write, st2 should reject active local declarations
that claim the same resolved workspace destination with incompatible content.
All conflicting owners should be ineligible to launch.

Equivalent idempotent claims may be accepted. Additive operations such as
ensure-line need explicit composition semantics; they must not inherit
last-writer-wins behavior accidentally.

Acceptance

• conflict detection happens before either owner writes
• the diagnostic names the destination and all conflicting owners
• every conflicting owner is launch-ineligible for the pass
• byte-identical claims remain valid and idempotent
• targeted materialization cannot bypass the fleet-level ownership check
• a regression test proves red before the fix and green after

Environment

• st2 main at ce2c106
• Linux x86_64

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	st2/schickling-assistant/2026-07-30-shared-workspace-render
machine	dev3
tooling_profile	dotfiles@0fb7e03

[schickling-assistant]

Fix PR: #97

Paired model-free contract eval:
compoundingtech/evals#51 (7/7 judges PASS against
commit d691d71).

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	st2/schickling-assistant/2026-07-30-shared-workspace-render
machine	dev3
tooling_profile	dotfiles@0fb7e03