feat(errors): AuthmuxError taxonomy + --json on 5 core commands (N3)

[NagyVikt]

Summary

Theme N3 from docs/future/17-ROADMAP.md — Error taxonomy + --json parity for the five core commands.

• New AuthmuxError base class with stable code, severity, hint, details, toJSON() (per 01-ARCHITECTURE.md §6.2). Every class in src/lib/accounts/errors.ts (AuthFileMissingError, AccountNotFoundError, NoAccountsSavedError, InvalidAccountNameError, AccountNameInferenceError, SnapshotEmailMismatchError, PromptCancelledError, InvalidRemoveSelectionError, AmbiguousAccountQueryError, AutoSwitchConfigError) now extends it with a stable E_* code. CodexAuthError is kept as a back-compat subclass so existing instanceof CodexAuthError catches still work.
• --json flag added to list, current, status, use, save. Output is a single JSON object on stdout; nothing else. Interactive prompts (update offer, account picker) are skipped under --json so stdout stays parseable.
• Central error handler in BaseCommand now routes AuthmuxError to the envelope under --json and exits with the §6.3 exit code.
• src/tests/error-taxonomy.test.ts enforces the §6.2 code allowlist, the severity table, CodexAuthError back-compat, the toJSON() shape, and the exit-code table.

Per N3 Risks: human-readable message strings are unchanged in this round. Scripts that grep stdout will keep working.

JSON envelope

// success
{ "ok": true, "data": { /* command-specific payload */ } }

// error
{ "ok": false, "error": { "code": "E_ACCOUNT_NOT_FOUND", "severity": "fatal",
                          "message": "...", "hint": "...",
                          "details": { "name": "alice" } } }

Exit codes follow §6.3: 3 E_AUTH_MISSING, 4 E_ACCOUNT_NOT_FOUND, 5 E_SNAPSHOT_EMAIL_MISMATCH, 6 E_REGISTRY_LOCKED, 7 E_REGISTRY_CORRUPT, 8 E_PROVIDER_NOT_INSTALLED, 64 E_PROMPT_CANCELLED, 1 generic.

Exit-criteria boxes (Theme N3)

• AuthmuxError class lands with code, severity, hint, details, and toJSON().
• Every class in src/lib/accounts/errors.ts extends AuthmuxError with a stable code per §6.2.
• The five core commands (list, current, status, use, save) accept --json and write a single JSON object to stdout.
• Exit codes follow the table in §6.3.
• src/tests/error-taxonomy.test.ts enforces the code/severity table.

Test plan

• npm run build — clean
• npm test — 89/89 pass (4 new, 85 prior)
• node dist/index.js list --json | JSON.parse(...) — single valid envelope
• authmux use does-not-exist --json emits E_ACCOUNT_NOT_FOUND envelope and exits 4
• authmux list --help shows the --json flag
• CI matrix (Ubuntu/macOS/Windows × Node 18/20/22) — pending on PR

Coordination note

Sibling PRs from N1 (durability — touches account-service.ts, registry.ts) and N4 (paths deprecation — touches src/lib/config/paths.ts and several command callers) are running in parallel. Expect a rebase if they land first; the conflict surface is small (this PR only touches command files at the parse(...) boundary, not paths.ts or registry.ts).

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