daemon: sanitize invalid UTF-8 in proto responses

[proboscis]

Summary

• Added sanitizeUTF8/sanitizeUTF8Slice helpers using strings.ToValidUTF8.
• Sanitized proto string fields in daemon responses where invalid UTF-8 can propagate (issue/run conversion + run issue metadata + capture/diff/daemon-log response payloads).
• Added socket-level regression tests that exercise real proto request/response framing to prevent EOF regressions when invalid UTF-8 is present.
• Added unit tests for UTF-8 sanitization helper behavior.

Acceptance Criteria Evidence

1) orch ps works even when issues contain invalid UTF-8

Repro run against a live daemon with an issue file containing invalid bytes (0xFF) in title/body:

$ orch --issues-root <tmp-vault> ps --issue utf8-bad --json
{
  "ok": true,
  "items": [ ... ]
}

2) orch monitor displays runs correctly

Monitor consumes daemon ListRuns/ListIssues proto APIs. Verified those APIs over real socket framing with invalid UTF-8 payloads:

$ go test ./internal/daemon -run 'TestListRunsAPI_InvalidUTF8IssueMetadataDoesNotBreakProto|TestListIssuesAPI_InvalidUTF8TextFieldsAreSanitized' -count=1 -v
--- PASS: TestListRunsAPI_InvalidUTF8IssueMetadataDoesNotBreakProto
--- PASS: TestListIssuesAPI_InvalidUTF8TextFieldsAreSanitized

This is the exact daemon boundary that previously failed marshal and caused client EOF.

3) Unit test for sanitizeUTF8

$ go test ./internal/daemon -run 'TestSanitizeUTF8' -count=1 -v
--- PASS: TestSanitizeUTF8_ValidStringUnchanged
--- PASS: TestSanitizeUTF8_InvalidStringBecomesValid
--- PASS: TestSanitizeUTF8Slice

Validation

• go build ./...
• go test ./internal/daemon -count=1
• go test ./test/integration -run TestPs -count=1
• make lint
• Note: go test ./... currently reports a pre-existing failure in internal/cli (TestApplyConfigDefaultsFallbacks, config schema EOF), unrelated to this daemon change.

Refs: orch-438