docs(bench): record the Phase 2 trial of code and branch reads

[tonyketcham]

Summary of changes

This is layer 14 of a stack. It sits on #293.

This PR adds no code. It records the paid trial of the Phase 2 reads from #292: proof find --path and proof find --refs all.

Haiku answered the 22 code-context and 22 branch questions twice on the git test bed from #293. Each session worked in its own clone and could use read-only git. The no-brief arm could therefore run git log, git show <ref>:<path> and git diff main...origin/<branch>. The findcode arm also had --path and --refs.

Measure	findcode	No-brief
Correct, of 88 question-runs	85	84
Only this arm right	2	1
Mean read tokens	11,642	26,283
Mean tool calls	7	13
Code set: correct, read tokens	42, 10,186	41, 29,912
Branch set: correct, read tokens	43, 13,097	43, 22,654

What it shows. Both arms are near the ceiling. On a graph of 524 records and about 40 source files, git and grep reach every answer. The trial cannot show an accuracy gain. What the new reads change is cost: 56% fewer read tokens and half the tool calls, on both question sets.

The 10x test showed that accuracy separates only once crawling stops scaling. The same test is needed here before claiming more than the saving: a test bed with the 10x graph, a larger codebase and more branches.

Files:

• four run files;
• a README section, "Phase 2 trial: code context and branches";
• a measurement Finding in .flatbread-proof/.

The compare.mjs --repo fix needed to score these runs correctly is in #293.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• I added doc comments to any new public exports, and inline comments to any hard-to-understand areas (none added)
• My changes generate no new console errors locally
• If applicable, try to include a test that fails without this PR but passes with it (results only, no code)

Does this introduce any non-backwards compatible changes?

• Yes
• No

Does this include any user config changes?

• Yes
  • If so, I have updated the relevant areas of documentation
• No

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