fix(runtime): route runtime filesystem commands by resolved execution host (#18325)

`ResolvedRuntimeFileTarget` carried `connectionId?: string` and no host id, so
`undefined` spelled three different answers at once — "runtime: host", "unresolved"
and "genuinely local". Its sole resolver read `store.getRepo(worktree.repoId)?.connectionId`
and never looked at `worktree.hostId`, which outranks every repo row, so one
arbitrarily chosen row decided the execution host for ~30 filesystem dispatches.
This is #18307's defect in the same file family; it was deliberately left out of
that PR rather than doubling an already-36-site diff.

The target now carries `executionHostId: ExecutionHostId` (never null, never
optional), resolved through `resolveWorktreeHostRouting` — the same adapter #18307
added — and dispatched through #18296's `resolveFilesystemRouteForHost`. Dispatch
sites call `requireRuntimeFileProvider`, where `null` means exactly one thing: the
host is `local` and the read happens here.

Four answers that used to collapse into one:

- `ssh:x` with a rival row on `ssh:y` — routes to x. Previously the first row won.
- `local` with a surviving `connectionId` — a row contradicting itself; no SSH
  connection is handed out.
- `runtime:<env>` — throws `ExecutionHostNotDispatchableError`. Its repo row's
  connection names a target in the *server's* namespace; reading it here reaches a
  same-named target on this client.
- rival rows disagreeing with no worktree host — `worktree_execution_host_unresolved`,
  matching the launch and Git paths rather than guessing a row.

Two further reads stop degrading. `assertRuntimeFileMutationExpectation` recomputed
the host from `connectionId`, so a client's host expectation could pass against a
host the workspace never named; it now compares the resolved host. And the
cross-workspace terminal tap coalesced `knownWorkspaceTarget?.connectionId ??
connectionId`, so a sibling workspace resolved as `local` inherited the origin
worktree's SSH target and statted a local path on the remote box; a non-optional
host id replaces rather than coalesces.

An unreachable SSH host still throws `SSH_FILESYSTEM_PROVIDER_UNAVAILABLE_MESSAGE`;
loss of contact is never evidence of locality (docs/reference/ssh-execution-boundary.md).
Quick-open listing and path search keep degrading to empty for an unreachable host —
that is a false negative, not a local answer — and now do so only for a host that
really is remote.

The whole `runtime-file-commands-*` family carries `@ts-nocheck` from a mechanical
class split, so removing the field could not raise the compile errors that made
#18307 safe. `runtime-file-command-target.ts` is deliberately checked, and a ratchet
test stands in for the errors the family cannot produce.

No wire change: `ResolvedRuntimeFileTarget` is main-process internal, and the SSH
watcher-release and grant keys are byte-identical to before.

src/main/runtime/runtime-file-command-host.ts

@@ -3,7 +3,7 @@ import type { Store } from '../persistence'
 import type {
   ResolvedRuntimeFileTarget,
   ResolvedRuntimeFileWorktree
-} from './runtime-file-watcher-leases'
+} from './runtime-file-command-target'
 import type { ExecutionHostId } from '../../shared/execution-host'
 import type { RuntimeNativeChatFileContext } from '../../shared/runtime-types'
 import type { FsChangeEvent } from '../../shared/filesystem-entry-types'
@@ -42,8 +42,9 @@ export type RuntimeFileCommandHost = {
     pathText: string,
     absolutePath: string
   ): boolean | Promise<boolean>
-  // `executionHostId`, not `connectionId`: a repo row's connection cannot tell `runtime:` from
-  // `local`, and this contract must not re-introduce that spelling. See runtime-git-command-target.
+  // `executionHostId`, not `connectionId`, on both target contracts: a repo row's connection cannot
+  // tell `runtime:` from `local`, and neither may re-introduce that spelling. See
+  // runtime-git-command-target and runtime-file-command-target.
   resolveRuntimeGitTarget(
     selector: string
   ): Promise<{ worktree: ResolvedRuntimeFileWorktree; executionHostId: ExecutionHostId }>