feat(ask): Add failure detection with actionable guidance

[hummat]

Problem or Motivation

papi ask currently has basic output hygiene (--pqa-raw, -v/--verbose) but lacks proper failure detection. When PaperQA2 crashes, hangs, or encounters bad PDFs, users get unclear errors or silent failures.

Current state:

• --pqa-raw flag for full passthrough
• -v/--verbose enables raw output
• Default behavior suppresses PaperQA2 streaming logs

Missing:

• Crash loop detection
• Bad PDF identification
• Actionable error guidance

Proposed Solution

1. Detect common failure modes:

   • PaperQA2 crashes (traceback parsing)
   • Hanging/timeout (no progress for N seconds)
   • Bad PDF errors (corrupted, encrypted, scan-only)

2. Provide actionable guidance:

   • "Paper X has a corrupted PDF. Try: papi attach X --pdf /path/to/fixed.pdf"
   • "PaperQA2 timed out. Try: papi ask --pqa-llm <faster-model> or add --verbose to debug"
   • "Install missing dependency: pip install paperqa[...]"

3. Exit codes:

   • 0: success
   • 1: query failed but recoverable (bad paper, suggest fix)
   • 2: unrecoverable error (missing deps, bad config)

Alternatives Considered

• Rely on --pqa-raw for debugging — requires manual log parsing, not user-friendly
• Wrapper script — moves complexity outside papi, harder to maintain

Area

RAG backends (PaperQA2/LEANN)

[hummat]

Cross-reference: #55.3

The security audit in #55 (finding 3) identified a bug in the existing crash recovery path (cli/ask.py:710-732). The guard logic is correct but depends on parsing PaperQA2's stdout output. Any future work on failure detection here should account for that finding — particularly if expanding the crash-doc parsing to handle more failure modes.