SDK: expected errors cannot carry extension-defined structured diagnostics (PmCliErrorContext is closed) #1631

Problem

createPmCliExpectedError(message, { context }) and new PmCliError(message, exitCode, context) take PmCliErrorContext, which is a closed interface. It lists only host fields: code, value, required, why, verification_errors, policy_violations, lease, recovery and so on. An extension can't attach its own structured diagnostics to a typed refusal, so agents get those diagnostics only as prose in message.

Real case

pm-changelog refuses a release whose requested version has no changelog section (unbraind/pm-changelog#302). The standalone CLI returns a selection_report object (filters, stage counts, excluded counts) in its JSON error. That's what an agent needs to decide whether to close items, set release, or pass --include-empty. The same refusal through the extension command (pm changelog generate) can't carry that object:

throw new PmCliError(msg, EXIT_CODE.GENERIC_FAILURE, { code: error.code, version: error.version, report: error.report });
// TS2353: Object literal may only specify known properties, and 'version' does not exist in type 'PmCliErrorContext'.

The workaround spreads the facts over unrelated fields: value for the version, required for the remedy, and verification_errors: ["excluded_status=1", ...] for the counts. That works, but the field names don't match their meaning, and nested structure is lost.

Proposal

Add one namespaced, JSON-serializable slot for extension diagnostics, for example:

interface PmCliErrorContext {
  /** Extension-owned structured diagnostics, rendered as-is in --json errors. */
  details?: Record<string, JsonValue>;
}

The host would render it in --json/TOON error output and apply its usual size budget and redaction. Telemetry and Sentry should ignore it, so extension payloads can't leak user data into crash reports.

Why

pm's goal is context management for agents. A typed refusal is most useful when its machine-readable facts reach the agent. Today only the host's own refusals can do that.