feat(jsonrpc): add stdio server for orchestrators

[lucas-p-costa]

Summary

Adds a JSON-RPC server over stdio as a subcommand of the main Forge binary:

forge json-rpc --directory <workdir>

This provides a structured integration surface for external orchestrators without requiring them to parse human-readable terminal output.

This PR builds on the direction explored in #2896, but was implemented as a clean port on top of current main, keeping JSON-RPC as a subcommand of the main forge binary.

What changed

• Adds forge_jsonrpc crate.

• Adds forge json-rpc --directory <workdir>.

• Adds discovery methods:

  • rpc.methods
  • rpc.discover
  • get_methods

• Adds conversation.create.

• Adds chat.stream subscription.

• Invalid/missing/malformed conversation IDs return JSON-RPC errors immediately instead of hanging.

• Adds include_reasoning?: boolean, defaulting to false.

• Emits explicit stream completion as {"type":"complete"}.

• Keeps normal CLI behavior unchanged.

• Does not touch provider/model code.

Validation

• cargo build

• cargo test -p forge_jsonrpc

• ./target/debug/forge json-rpc --help

• Normal CLI smoke:

  • ./target/debug/forge -C /tmp/forge-multica-smoke-2 -p "Reply with exactly: cli-still-ok"

• JSON-RPC smoke:

  • conversation.create returns a UUID
  • chat.stream emits message chunks and complete
  • default chat.stream emits 0 reasoning events
  • include_reasoning: true emits reasoning events
  • invalid conversation ID returns JSON-RPC error without timeout

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[lucas-p-costa]

Thanks for the reminder. This PR is still active and ready for review.

I opened it as a clean follow-up to the earlier JSON-RPC stdio direction in #2896, but adjusted it to keep JSON-RPC as a forge json-rpc subcommand of the main binary rather than adding a second binary.

A few implementation notes:

• no provider/model code is touched;
• normal CLI behavior is unchanged;
• conversation.create and chat.stream are exposed through JSON-RPC;
• invalid/missing/malformed conversation IDs return JSON-RPC errors instead of hanging;
• include_reasoning defaults to false;
• stream completion is emitted explicitly as {"type":"complete"};
• cargo build and cargo test -p forge_jsonrpc pass locally.

Happy to split this into smaller PRs or adjust the API shape if there is a preferred direction.

[github-actions]

Action required: PR inactive for 5 days.
Status update or closure in 10 days.