fix(acp): honor per-session workspace roots

[dawn (dawNotPoi)]

Problem

ACP clients provide a workspace in session/new.cwd, but the server ignored it and reused one agent/backend per configured agent. Filesystem operations could therefore run against the server process workspace, and concurrent sessions could share the wrong backend session ID.

Fix

• Store a workspace root on each session, falling back to the server default.
• Create the agent and filesystem backend per session so roots and ACP session IDs remain isolated.
• Let the default CLI configuration use the server-selected per-session backend, including ACP filesystem capabilities when available.
• Use the session root for filesystem backends and tool-call locations.
• Add a patch changeset and regression coverage for explicit cwd, fallback behavior, ACP-backed sessions, CLI behavior, and two independent workspaces.

Test

• pnpm --filter deepagents build
• pnpm --filter deepagents-acp build
• pnpm --filter deepagents-acp test:unit (160 passed)
• pnpm --filter deepagents-acp test:int (33 passed)
• oxlint and oxfmt checks on the changed ACP files

The standalone package typecheck still reports the pre-existing ACP SDK/backend type incompatibilities also present on main; this change adds no new diagnostics.

Closes #654

[changeset-bot]

🦋 Changeset detected

Latest commit: 06bb73c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
deepagents-acp	Patch

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commit: 06bb73c

[corridor-security]

The PR honors per-session workspace roots by accepting params.cwd from the ACP client, which expands the filesystem backend and terminal root to any client-supplied path; because ACP uses a stdio transport connecting a local IDE, this is a lower-severity concern than a typical network-facing path traversal, but remains a valid issue for deployments where the server is shared or the client is not fully trusted.