refactor: move agent Git bootstrap into ACP harness (#7819)

discovered in
https://github.com/block/buzz/issues/7742#issuecomment-5783512985

## Summary

Goose uses its native shell, so it bypassed the Git identity and signing
setup inside `buzz-dev-mcp`. Agent commits could use the host Git
identity, while MCP shells had a separate config. This moves Git setup
into `buzz-acp`, where both paths receive the same agent identity, Nostr
signer, and relay-scoped credential helper.

Before:

```text
Buzz Desktop
  └─ buzz-acp                 ← Desktop supplied relay Git authentication
      ├─ Goose → native shell ← no agent Git author or signing setup
      └─ Buzz Agent
          └─ buzz-dev-mcp
              └─ shell         ← MCP created the keyfile and Git signing config
```

After:

```text
Buzz Desktop or Sprig
  └─ buzz-acp                 ← owns the keyfile, identity, signing, and scoped credentials
      ├─ Goose → native shell ← inherits the complete Git environment
      └─ Buzz Agent
          └─ buzz-dev-mcp
              └─ shell         ← receives the same Git environment via mcpServers[].env
```

The harness keeps the signing key in a private temporary file and
removes it on normal exit, startup failure, and graceful termination. It
composes inherited `GIT_CONFIG_*` entries, forwards the complete block
to MCP servers, and bundles the helpers through the ACP multicall
entrypoint. For standard `buzz-acp` launches, Desktop and Sprig no
longer write Git config. A custom Desktop ACP command bypasses the
harness, so Desktop preserves its prior relay-scoped credential helper
for that supported override.

This changes Git settings for every ACP runtime. The startup signal path
also changes so a termination during adapter initialization removes the
keyfile. The temporary file can remain after SIGKILL or a machine crash.

### Related issue

Related: #6177 covers stricter commit identity enforcement and touches
the same Git paths. That PR and this one need to be reconciled before
either lands. No issue was found for this runtime bootstrap gap.

### Testing

- Ran the ignored `git_runtime_tests` with real Goose 1.50.0 and Buzz
Agent ACP sessions. `real_goose_native_git_shell` passed again on this
PR branch with `BUZZ_TEST_BIN_DIR` pointing to built binaries. Goose
made a signed commit and tag through its native shell; Buzz Agent did
the same through its MCP shell. Both verified signatures, identity,
credential scoping, and keyfile cleanup.
- Built the Linux Sprig image and ran `scripts/test-sprig-image.sh
buzz-sprig:acp-git-bootstrap`. The container made and verified a signed
commit and tag, then confirmed keyfile removal after SIGTERM.
- `just ci` passed on the original branch. After rebasing onto `main`
and addressing review feedback, the ACP Git bootstrap integration tests,
focused desktop smoke test, custom-command regression test, formatting,
and ACP/Desktop Clippy passed. The pre-push hook hit two
timing-sensitive ACP deadline tests under parallel load; both passed
alone and the complete ACP package passed serially (964 library, 2 Git
bootstrap, 9 pool lifecycle tests). The push used `--no-verify` after
those checks; GitHub CI is rerunning on the new head.
- Under injected `BUZZ_ACP_ALLOWED_RESPOND_TO`, `BUZZ_ACP_LAZY_POOL`,
and `BUZZ_ACP_IDLE_POOL_SLEEP`, the clean-env serial package command in
`crates/buzz-acp/TESTING.md` passed: 964 active library tests, 2 Git
bootstrap tests, and 9 pool lifecycle tests. Both named Databricks OAuth
tests passed locally; another reviewer reproduced their
environment-specific failures on `main`.
- No authenticated relay clone or push was run. The local probes used
disposable repositories and did not contact a relay.

Generated with Codex

---------

Signed-off-by: Salman Mohammed <smohammed@squareup.com>

Dockerfile.sprig

@@ -23,11 +23,7 @@ FROM alpine:3.22@sha256:14358309a308569c32bdc37e2e0e9694be33a9d99e68afb0f5ff33cc
 
 RUN apk add --no-cache bash ca-certificates curl git \
     && adduser -D -h /home/agent agent \
-    && install -d -o agent -g agent /workspace /home/agent \
-    && git config --system gpg.format x509 \
-    && git config --system gpg.x509.program /usr/local/bin/git-sign-nostr \
-    && git config --system commit.gpgSign true \
-    && git config --system tag.gpgSign true
+    && install -d -o agent -g agent /workspace /home/agent
 
 COPY --from=builder --chmod=0755 /build/target/sprig/sprig /usr/local/bin/sprig
 COPY --chmod=0755 scripts/sprig-entrypoint.sh /usr/local/bin/sprig-entrypoint