feat(server): typed no-execution-plane environment for workerd profile

[kitlangton]

What

Complete the workerd profile's capability story after #41918 and #42076: a bare location now has a typed no-execution-plane process spawner instead of profile-level process stubs.

Shell and stdio MCP route through the location Environment. Bare locations fail each operation with a clear location-specific error; attaching a workspace driver automatically supplies the execution plane without changing the profile.

How

• Add EnvironmentUnavailable in packages/core/src/environment/unavailable.ts. Its ChildProcessSpawner fails with Effect's typed PlatformError channel and the message: This location has no execution plane: no workspace is attached and the host cannot spawn processes.
• Replace the workerd profile's cross-spawn implementation with that layer and remove the hand-written Shell.Service replacement, allowing the standard location-scoped Shell service to build.
• Settle Shell spawn failures through Shell.create instead of swallowing them in its managing fiber.
• Lazily initialize Shell output storage only after a process spawns, so bare workerd locations do not import filesystem modules or create unused output directories.
• Retire the mcp.stdio capability flag from core, server options, routes, and the workerd profile. Local MCP servers now attempt connection through Environment.spawner and record the no-plane error as failed.
• Add core coverage for the typed spawner and MCP failed status, plus a real-workerd regression proving a failed Session shell operation releases session state and permits a later turn.

flowchart TD
L[Location] --> W{workspace attached?}
W -->|yes| D[Workspace driver]
W -->|no| H[Host spawner]
H -->|workerd| N[No-execution-plane spawner]
D --> E[Environment.spawner]
N --> E
E --> S[Shell]
E --> M[stdio MCP]
N --> F[Typed per-operation PlatformError]

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Scope

• This follows the workerd profile from feat(server): workerd runtime profile and SDK workerd entrypoint #41918 and stdio MCP Environment routing from refactor(core): spawn stdio MCP servers through the location environment #42076; both are already merged into v2.
• The workerd shell tool still encounters the profile's existing empty-WASM ShellParse implementation before Shell.create. This PR does not bypass command parsing or permission analysis; direct Shell service operations and every consumer that reaches Environment.spawner get the typed fallback.
• Existing FileSystem, FileSystemSearch, and Pty profile replacements are unchanged because they have separate host dependencies and capability seams.

Testing

• bun install
• bun typecheck in packages/core
• bun typecheck in packages/server
• bun run test test/mcp.test.ts test/environment.test.ts test/tool-shell.test.ts in packages/core (69 pass, 9 skip)
• bun run test in packages/workerd-spike (6 pass, real workerd isolate)
• bun run probe:workerd in packages/server (bundle probe OK, 29.9 MiB)
• Push hook: bun turbo typecheck --concurrency=3 (32 tasks passed)