prompts: require proof for missing kernel symbols

[OllieinCanada]

Fixes #439

Summary

Require concrete source and build evidence before a kernel review reports that a referenced symbol does not exist.

Root cause

The git_grep summary added for #357 makes whole-tree matches visible, but #439 shows that presentation alone is not sufficient: the reviewer still claimed two mbcache functions were absent despite clear declarations, definitions, exports, and callers in the supplied results.

This adds a narrow verification boundary to the existing kernel false-positive guide:

• search the exact identifier across the complete review worktree
• inspect declarations, definitions, and exports before asserting absence
• distinguish a genuinely missing symbol from a missing include, Kconfig/Makefile exclusion, or module-linkage problem
• require the exact configuration and build/link path for configuration-dependent findings
• treat an unavailable, failed, or incomplete search as insufficient evidence rather than proof of absence

The output checklist makes the evidence auditable without globally suppressing real undefined-symbol findings.

Scope

Only third_party/prompts/kernel/false-positive-guide.md changes. The grep tool, review pipeline, prompt bundle code, tests, and REVISION file are unchanged. No LLM-backed regression test is added.

Validation

• manually checked the positive and negative reasoning boundaries against the False positive HIGH issue "Build failure due to hallucinated function ext4_dir_rec_len(1, NULL)" #357 and False positive LOW issue: undefined calls to mb_cache_entry_delete_or_get() and mb_cache_entry_wait_unused() #439 examples
• cargo fmt --all -- --check passes
• git diff --check passes
• the cumulative diff contains exactly one prompt file