Fix error handling in boss-mode report generation to surface I/O errors

[Copilot]

Boss-mode report generation silently fell back to the full LLM pipeline on any file read error, including permission/I/O failures. This masked real errors and wasted tokens.

Changes

• Error handling: Explicitly check for os.IsNotExist vs other errors
  • File not found → log and continue to full pipeline (expected)
  • Permission/I/O errors → surface to user and abort (unexpected)

// Before: all errors treated as "file doesn't exist"
if content, readErr := os.ReadFile(teamReportPath); readErr == nil && len(content) > 0 {
    // use shortcut
}
log.Printf("no existing team report found, running full pipeline")

// After: distinguish expected from unexpected errors
content, readErr := os.ReadFile(teamReportPath)
if readErr != nil {
    if !os.IsNotExist(readErr) {
        log.Printf("Error reading team report file %s: %v", teamReportPath, readErr)
        postEphemeral(api, cmd, fmt.Sprintf("Error reading team report file: %v", readErr))
        return
    }
    log.Printf("no existing team report found, running full pipeline")
} else if len(content) > 0 {
    // use shortcut
}

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.