docs: agent platform architecture plan — augmentation + task breakdown (#173)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

docs/plans/chroot-sandboxing.md

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+# Chroot Agent Sandboxing — Process Isolation for Agent Tool Execution
+
+> **Status:** Stub — deferred. Referenced from `2026-03-15-agent-platform-architecture.md` (Phase 7 Workspaces → Chroot Agent Sandboxing).
+> Implement after Workspaces (P8-015) is complete. Requires workspace directory structure and `WorkspaceService` to be operational.
+
+**Date:** 2026-03-15
+**Packages:** `apps/gateway`
+
+---
+
+## Problem Statement
+
+Agent sessions can use file, git, and shell tools. Path validation in tools is defense-in-depth but insufficient alone — an agent with shell access can run `cat /opt/mosaic/.workspaces/other_user/...` and bypass gateway RBAC.
+
+Chroot provides OS-level enforcement: tool processes literally cannot see outside their workspace directory.
+
+---
+
+## Design (Sweet Spot)
+
+Chroot strikes the balance between full container isolation (too heavy per session) and path validation only (escape-prone):
+
+- Gateway spawns tool processes inside a chroot rooted at the session's `sandboxDir`
+- Requires `CAP_SYS_CHROOT` capability on the gateway process (not full root)
+- Chroot environment provisioned by `WorkspaceService` on workspace creation (minimal deps: git, shell utils, language runtimes as needed)
+- Alternative for Docker deployments: Linux `unshare` namespaces (lighter, no chroot env setup)
+
+---
+
+## Scope (To Be Designed)
+
+- [ ] Chroot environment provisioning — `WorkspaceService.provisionChroot(workspacePath)` on project creation
+- [ ] Minimal chroot deps — identify required binaries/libs per tool type (file: none; git: git binary; shell: bash, common utils)
+- [ ] Gateway capability — document `CAP_SYS_CHROOT` requirement; Dockerfile and docker-compose.yml changes
+- [ ] Tool process spawning — modify `createShellTools`, `createFileTools`, `createGitTools` to spawn via chroot wrapper
+- [ ] Docker alternative — `unshare --mount --pid --user` namespace wrapper as fallback for environments without chroot capability
+- [ ] Defense-in-depth layering — chroot + path validation both active; neither alone is sufficient
+- [ ] Chroot cleanup — integrate with `SessionGCService` / workspace deletion
+- [ ] AppArmor/SELinux profiles (v2) — restrict gateway process file access patterns for multi-tenant hardening
+
+---
+
+## Security Constraints
+
+- What lives **inside** the chroot (agent-accessible): workspace files, git repo, language runtimes
+- What lives **outside** the chroot (gateway-only, never agent-accessible): Valkey connection, PG connection, other users' workspaces, gateway config, OTEL endpoint, credentials
+
+---
+
+## Dependencies
+
+- Workspaces (P8-015) — chroot is rooted at workspace directory; workspace must exist first
+- Tool hardening (P8-016) — path validation stays active as defense-in-depth alongside chroot
+
+---
+
+## References
+
+- Original design context: `docs/plans/2026-03-15-agent-platform-architecture.md` → "Chroot Agent Sandboxing" section
+- Current tool implementations: `apps/gateway/src/agent/tools/`