fix: actor set-value KEY with no value doesn't delete the record

[kuntal1461]

Fixes #1193.

What changed and why

• src/commands/actor/set-value.ts: replace value || process.stdin with value ?? cachedStdinInput

  • || coerced both undefined (omitted arg) and "" (explicit empty) to the already-drained process.stdin stream, so the deletion check on lines 64–70 never fired
  • ?? (nullish coalescing) preserves "" as a deletion sentinel and passes undefined through to cachedStdinInput for the piped-stdin case
  • process.stdin is drained by readStdin() at entrypoint load time; cachedStdinInput is the correct pre-buffered source (same pattern used by actor push-data)

• test/e2e/commands/actor/set-value.test.ts: add regression tests for the two broken cases

  • delete record when no value or stdin is provided
  • set record from piped stdin
  • strengthen existing set-value test to assert file content

Regression introduced by

Commit 28959631 (#760, "move away from oclif") removed ignoreStdin: true from the args but did not update the process.stdin fallback to use cachedStdinInput.

Install size impact

No new dependencies. cachedStdinInput is already exported from src/entrypoints/_shared.ts.