fix: show the implementer the repository it is patching

[thedancingdeveloper]

Closes #135. Substantially mitigates #133.

The bug

IMPLEMENT_PROMPT asks for a diff that "applies cleanly at the repository
root". context_provider defaulted to lambda _record: "", and
agent-harness run never passed one — so Repository context: was always
empty and the model was asked to patch files it had never seen.

It cannot write context lines it does not know, so it wrote hunks with none:

--- a/calc.py
+++ b/calc.py
@@ -0,0 +1,4 @@
+def multiply(a, b):
+    return a * b

git apply refuses that against a non-empty file. --unidiff-zero accepts it
and inserts at line 1 — which is how multiply ended up above the module
docstring, and two tests above from calc import add, subtract, with pytest
green throughout.

Two parts to the fix

Show it the repository. A default context_provider giving the tracked
file listing, files the brief names in full, then the rest smallest-first
within a 60k budget. Binary files are listed, never read.

Read from the base ref, not the working tree. This one is subtle and was
found by the first fix not working. The tree still holds the previous item's
branch when the implementer is called — the branch for this item is not cut
until the apply step — so the model was shown a file its patch would never
meet, and every patch failed to apply. The base is now resolved before the
implementer runs and read via git show <base>:<path>. Correct by
construction, and it touches no checkout.

Live result

Same three-item backlog, same endpoint, across the three fixes in this series:

	outcome
before #132	0/3 — all rejected on a plumbing artefact
after #132	1/3 — the other two rejected, correctly, for misplacement
after this	3/3 completed

Each item on its own branch, tests passing on each independently (3, 4 and 4
tests), the stacked item correctly based on its dependency, and calc.py
ending up exactly as a human would write it — docstring first, functions in
order.

On #133

Not closed. With real context the model produces proper diffs and the first
rung of the ladder succeeds, so --unidiff-zero stops being reached in
practice. The rung can still misplace a zero-context hunk if one arrives; that
remains worth addressing on its own terms.

Tests: four new, covering the listing reaching the implementer, a named file
being included whole under a tight budget, an empty repository, and binary
files being listed but not read. Suite, ruff and mypy . green.