feat(leann): Add grep fusion for hybrid retrieval

[hummat]

Problem or Motivation

LEANN supports use_grep=True in its Python API for exact text matching, but this isn't exposed in papi. For technical papers, exact string hits matter: hyperparameters ("λ=0.1"), equation references ("Eq. 7"), dataset names, specific values.

Pure vector search can miss these precise matches, reducing recall on technical queries.

Proposed Solution

Expose LEANN's grep search capability for hybrid vector + exact-match retrieval:

papi ask --backend leann --leann-grep "query"

This would fuse grep + vector results for better recall on technical queries.

Alternatives Considered

• Manual grep before querying — cumbersome, doesn't fuse with vector results
• Use higher top_k — increases noise without targeting exact matches

Area

RAG backends (PaperQA2/LEANN)

Blockers

Blocked on upstream: LEANN's leann ask CLI does not expose --use-grep yet. Needs PR to LEANN first.

[hummat]

Status: Blocked on upstream LEANN CLI

The installed LEANN CLI does not currently expose a --use-grep flag. The paperpipe code that previously passed --leann-grep → --use-grep through to LEANN has been removed, and a runtime guard now rejects these flags with an actionable error message explaining the limitation.

What was done:

• Removed --leann-grep click option and leann_grep parameter from _ask_leann()
• Added guard in ask.py that rejects --leann-grep and --use-grep passthrough with a clear UsageError
• Tests verify the flags are absent from --help and rejected at runtime

What's needed to unblock:

• Upstream LEANN CLI needs to ship --use-grep support
• Once available, re-add the --leann-grep option and remove the guard