Context: solidify Cody keyword context

[jtibshirani]

This issue tracks an initial round of context quality improvements based on our manual evaluation. It focuses on the core keyword retrieval algorithm, and only considers single-repo context.

Quality improvements

• https://github.com/sourcegraph/sourcegraph/pull/61570
• https://github.com/sourcegraph/sourcegraph/pull/61573
• Debug why we perform poorly on "Changelog" and "Ownership" searches
  • https://github.com/sourcegraph/sourcegraph/pull/61663
  • https://github.com/sourcegraph/sourcegraph/pull/61664
  • Ranking: include filename matches in bm25 zoekt#757

Bug fixes

• https://github.com/sourcegraph/sourcegraph/pull/61806
• https://github.com/sourcegraph/sourcegraph/pull/61848

/cc @sourcegraph/search-platform

[jtibshirani]

Summary of user-facing improvements:

• Squashed bug where a query like "what's going on?" would return random results
• Customers who use other docs formats like wiki now get docs results
• Small improvement on golden queries evaluation before and after these improvements. The main effect is that we are now better at Ownership queries

Before

Find symbol	7/9
Find string	2/2
Explain concept	4/5
Check dependency	1/2
Find logic	16/22
Gather information	6/9
Changelog	0/2
Ownership	0/2
Combined recall	36/53

After

Find symbol	7/9
Find string	2/2
Explain concept	4/5
Check dependency	1/2
Find logic	16/22
Gather information	6/9
Changelog	0/2
Ownership	2/2
Combined recall	38/53