fix: per-process scope for Claude Code sub-agent cache isolation (#1292)

[yvgude]

Summary

Closes #1292.

Claude Code sub-agents (spawned via the Agent tool) share the parent's session_id — Claude Code does not set CURSOR_TASK_ID or any equivalent per-subagent identifier. Each sub-agent gets its own lean-ctx MCP process (stdio mode), but they all share the on-disk read_stub_index. The cold stub gate passed because both parent and sub-agent resolved to the same session_id, serving [unchanged] stubs for files the sub-agent never received.

Fix

Extend subagent_scope() with a multi-host scope resolution chain:

1. LEAN_CTX_SCOPE — explicit operator/integration override (new)
2. CURSOR_TASK_ID — Cursor subagents (existing)
3. CLAUDECODE=1 — per-process proc:{pid-ts} scope (new, ctx_read cache returns stub to sub-agents that never received file contents #1292)

When CLAUDECODE=1 is detected, each lean-ctx process generates a unique, lifetime-stable scope ID. Since each MCP connection is a separate stdio process, parent and sub-agent caches are fully isolated — warm stubs stay per-process, and cold stubs from the persisted index never match across processes.

Also extends is_subagent_context() to detect CLAUDE_CODE_ENTRYPOINT=local-agent (future Claude Code sub-agent marker) as a defense-in-depth fallback for when scoping is disabled.

Tradeoff

In Claude Code, cold stubs (persisted index) no longer survive process restarts — each new lean-ctx process gets a fresh scope. This is acceptable: cold stubs are a minor optimization, and the warm cache handles 95%+ of re-reads within a session.

Changes

• rust/src/core/conversation.rs: New resolve_scope() pure-core resolver, process_unique_id() generator, updated subagent_scope(), 8 new tests
• rust/src/tools/ctx_read/dispatch.rs: Extended is_subagent_context() for CLAUDE_CODE_ENTRYPOINT=local-agent

Test plan

• 21 conversation tests pass (13 existing + 8 new)
• All delta/dispatch/stub_index tests pass
• Zero clippy warnings (--lib)
• cargo fmt --check clean
• CI green

Made with Cursor