feat(sdk-go): bind filesystem operations to explicit identity

[GodBlf]

Summary

Adds sandbox.FilesWithIdentity(uid, gid) so Go callers can bind file operations to an explicit Linux UID/GID.

Includes an identity-bound filesystem interface and client, regression tests, and Go documentation.

Bound clients preserve connection and endpoint-routing settings. Unsupported identity routes surface as errors without falling back to the default filesystem routes. Identity selection belongs in the trusted backend; this feature enforces Linux filesystem permissions and does not introduce a tenant isolation boundary.

Related to #1843. Follows core #2070, merged on September 30, 2026. This PR is independent of the other SDK follow-ups.

Testing

Local results for commit 2685c1832958d3d4d012eb384756d5f7be8d22d9 on merged-main base 04dd996ca2b72470d73ecfae24ca267addceb100.

• Not run

• Unit tests

• Separate live-sandbox integration run on this rebased head

• e2e / manual verification on this rebased head

• Linux Go 1.25.9: go test -count=1 ./... passed for all packages.

• pnpm docs:build passed on this rebased branch.

Real-sandbox client verification was not run for this language; coverage is from the unit/build checks above.

Breaking Changes

• None
• Yes

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs
• Added/updated tests
• Security impact considered
• Backward compatibility considered

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.