feat(core): add workspace environment foundation - #40967

What

Adds the workspace environment foundation as a pure addition. Nothing consumes this module yet.

The core thesis is that spawn is the driver contract: Files is derived from an Effect ChildProcessSpawner, while drivers may provide native fast-path overrides that promise identical observable semantics.

How

• Adds the Driver, Files, FileInfo, DirEntry, and exact NotFound / WrongKind / Failed error vocabulary under packages/core/src/environment/.
• Derives GNU/Linux file operations from a supplied spawner, with one bounded process round trip per intent, fused metadata reads and parent-creating writes, byte-precise ranges, and NUL-safe directory listing.
• Adds a complete Map-backed memory driver whose spawner fails and whose file overrides implement POSIX path and symlink semantics.
• Adds one shared conformance suite under packages/core/test/lib/. The suite is the executable parity mechanism for this driver and future local and Modal drivers.
• Runs the same suite against memory everywhere and against execDefaults plus CrossSpawnSpawner on Linux.

GNU-only defaults are intentional: these commands run inside sandbox images we control. The exec-default conformance leg is skipped on non-Linux development hosts and runs on Linux CI.

Scope

• Pure addition, consumed by nothing in this PR.
• No V1 changes.
• No watch support.
• No local or Modal native driver yet.
• The conformance helper stays in core test infrastructure until another package needs a promoted runtime export.

Testing

• cd packages/core && bun typecheck
• cd packages/core && bun run test test/environment*.test.ts (8 memory tests passed; 8 GNU tests skipped on macOS)
• cd packages/core && bun run test (1,555 passed, 15 skipped; unrelated existing failures in models.test.ts, invalid-regex handling in tool-search.test.ts, and config/plugin.test.ts)
• Push hook: workspace typecheck passed across all 32 participating packages.