Make the encoding refusal legible everywhere, and confine runCommand to the open workspace

[Seungpyo1007]

Two leftovers from the encoding work in #61 / #62.

1. The refusal was only legible inside the editor

The main process throws a code — SCHUTZ_ENCODING:<kind> — because it does not know the UI language. Only MonacoPane translated it, so the same error surfaced raw everywhere else.

The agent was the worst case: read_file returned 오류: SCHUTZ_ENCODING:not-utf8, which tells the model nothing, so it re-reads the same file and burns rounds. Proposal cards showed the raw code as the failure reason too.

src/encodingNote.ts now owns the mapping and all three call sites share it.

2. runCommand never checked the workspace

Every other fs handler runs assertRoot(root). runCommand only checked existsSync, so any path the renderer passed — including the user's home directory — was a valid cwd.

The comment above the spawn also claimed cwd confines the command to the workspace. It does not: with shell: true the command can cd or use absolute paths. The real boundary is the approval gate above it. Comment corrected rather than left to mislead the next reader.

Verification

• 9 unit tests for the shared mapping, including the IPC-wrapped Error invoking remote method ... prefix. 972 passing (was 963).
• Real app: encoding probe 7/7 (editor message unchanged after the refactor); replace + run probe 7/7 — echo inside the open workspace still runs and returns hi, the same call with cwd: C:/Users/29 is refused.
• npm run typecheck clean. main.cjs line endings unchanged (1886 CRLF / 0 LF).