fix(server): contain API session working directories

[Sayt-0]

Summary

• add a dedicated opt-in --session-workingdir-root boundary for session working directories supplied through the HTTP API
• canonicalize configured roots and candidate directories before rejecting traversal, sibling-directory, and symlink escapes
• preserve unrestricted local-daemon behavior when no root is configured, without treating --working-dir or the process cwd as an implicit security boundary
• document the deployment model and add server and CLI regression coverage

Security alert: https://github.com/docker/docker-agent/security/code-scanning/57

Alert expectations

Expectation	Implementation
Prevent untrusted API callers from selecting arbitrary host paths	WithSessionWorkingDirRoot constrains POST /api/sessions working directories when configured
Reject traversal and symlink escapes	Both root and candidate are resolved with filepath.EvalSymlinks; containment uses filepath.Rel and filepath.IsLocal
Avoid the behavior regression reverted by #3788	The default remains unrestricted and neither runConfig.WorkingDir nor the process cwd becomes a boundary
Support API and attached-run control planes	serve api exposes the flag; run --listen accepts the same hidden advanced flag
Keep the intentional unrestricted flow explicit to CodeQL	The remaining unrestricted sink has a rule-specific local suppression with the compatibility rationale

Validation

• task build
• task lint
• env -u HTTP_PROXY -u HTTPS_PROXY -u http_proxy -u https_proxy -u ALL_PROXY -u all_proxy task test
• npx --yes markdownlint-cli2@0.22.1 features/api-server/index.md features/cli/index.md
• ./scripts/docs-check-canonical.sh
• ./scripts/docs-check-llms-txt.sh
• offline documentation link check with Lychee

The proxy variables are removed for the test command because the local development proxy intercepts private-address SSRF fixtures; the same failing packages pass without that interception.

Security and compatibility notes

• Containment is opt-in so long-lived local daemons can continue opening arbitrary user-selected workspaces. Multi-user or network-exposed deployments should configure the new root.
• The configured root should be controlled by the operator. Filesystem namespace changes between canonicalization and later tool use remain subject to normal TOCTOU constraints when untrusted principals can mutate the root.
• Existing HTTP error classification is unchanged.
• Alert closure depends on the CodeQL analysis for this branch.

[aheritier]

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