fix(mage): summarize proto changes instead of pasting the whole diff

[josephschorr]

Fixes the one failure in the first real end-to-end regeneration (run 33014735194, PR #61). Java failed; the other six languages succeeded.

Cause

Each idiomatic client's Gen() pasted the entire raw git diff of its proto-client directory into the Claude prompt. Java's protobuf codegen emits one verbose file per message, so that diff is enormous:

proto client	files	changed lines
spicedb-java-proto	263	56,401
spicedb-csharp-proto	4	1,019
spicedb-python-proto	29	629
spicedb-go-proto	18	210
rust / typescript / ruby	4–5	39–113

Java is 55× the next largest, and the prompt blew the context window outright:

==> Invoking Claude to update idiomatic client...
Prompt is too long
Error: claude invocation failed: exit status 1
==> FAILED: spicedb-java: running "mage -d spicedb-java gen" failed with exit code 1

This is structural, not a flake. Java would fail on every regeneration, making every PR permanently regen-partial.

Fix

Summarize instead of pasting: git diff --stat plus git diff --name-status, with the prompt telling Claude to read the changed files itself. Claude has file access on the runner, so this is both smaller and more useful than a truncated diff — it can open whichever files it actually needs rather than working from a arbitrarily-cut fragment.

Applied uniformly to all seven spicedb-*/Magefile.go. proto-clients/* are untouched: they diff their own directory immediately after buf generate, so their prompts are small by construction.

Two details worth noting for review:

• The empty-change check is re-keyed to names. It previously tested the raw diff string. Getting this wrong in either direction would make every client always-skip or never-skip.
• Each file's own prompt text is preserved verbatim. The seven differ in language and test commands; only the change-delivery portion was touched.

Verification

go vet -tags mage clean at root and in the six vettable client dirs; gofmt clean; root mage tests pass; all seven files confirmed changed, with the names check and the claudeAvailable() gate present in each.

spicedb-go cannot compile with -tags mage — it holds package main and package spicedb in one directory. Pre-existing and unrelated; checked with gofmt instead.