Add Sprig all-in-one agent binary

[tlongwell-block]

Summary

Adds Sprig, a deploy-anywhere all-in-one binary for the Sprout ACP harness, agent, and developer MCP.

Sprig is a busybox-style multicall binary:

• sprig prints usage/version
• sprout-acp symlink → ACP harness personality
• sprout-agent symlink → ACP agent personality
• sprout-dev-mcp symlink → developer MCP personality
• existing dev-MCP helper multicall names (rg, tree, sprout, git-credential-nostr, git-sign-nostr) continue to dispatch through the dev-MCP path

This replaces the old sprout-agent-bundle release path with a Sprig release path:

• rolling release: sprig-latest
• tagged release: sprig-v*
• build script: scripts/build-sprig.sh
• workflow: .github/workflows/sprig.yml

Size

Compared on my host against the current bundle-style stripped binaries:

artifact	size
current 3 stripped binaries total	30,953,768 B / 29.52 MiB
profile-only optimized 3 binaries	14,984,296 B / 14.29 MiB
Sprig multicall binary	7,748,264 B / 7.39 MiB
Sprig tar.gz	4.7M

The size win comes from storing the shared Rust/Tokio/reqwest/rustls/aws-lc/etc. stack once instead of once per binary.

Implementation notes

• Factors sprout-acp, sprout-agent, and sprout-dev-mcp into library run() entrypoints plus thin existing binaries.
• Adds a new sprig crate that only dispatches by argv[0].
• Adds [profile.sprig]: opt-level = "z", FatLTO, codegen-units = 1, panic = "abort", strip = true.
• Uses symlinks in the tarball for command-name compatibility.
• Leaves runtime behavior per personality unchanged; no runtime unification or cfg-stripping.

Verification

Local checks run:

• cargo fmt --check
• cargo check -p sprig -p sprout-acp -p sprout-agent -p sprout-dev-mcp
• cargo test -p sprout-acp -p sprout-agent -p sprout-dev-mcp
• ./scripts/build-sprig.sh 0.1.0-test $(rustc host) and tar inspection
• sprig --help, bare sprig exits non-zero with usage, sprig --version
• extracted archive smoke: sprout-acp --help
• local relay E2E per TESTING.md:
  • just setup
  • release sprout-relay on 127.0.0.1:3030
  • CLI smoke: generated sender key, created channel, sent/read message, thread query
  • extracted Sprig archive
  • ran sprout-acp from archive, configured it to spawn sprout-agent and sprout-dev-mcp from archive
  • fake OpenAI-compatible provider requested dev__shell
  • final SPRIG_FINAL_OK reply came back through local relay

AI Assistance

• This PR was created with AI assistance.

Signed-off-by: Tyler Longwell tlongwell@squareup.com