Add meeting-chat workflow for AI-assisted pull request creation

## Summary
Enable Conclave meeting-chat participants to request code changes and have Conclave use an agent harness to implement the changes and open a pull request.

## Motivation
Participants should be able to move from a request made in meeting chat to a reviewable PR without leaving Conclave.

## Proposed behavior
- Add a chat-triggered workflow for creating a PR from a participant request.
- Use a supported coding-agent harness, evaluating options such as:
- OpenCode
- Codex TypeScript SDK
- Claude Code agent SDK
- The workflow should:
1. Collect the requested change and required repository context.
2. Run the selected coding agent in an appropriate repository workspace.
3. Have the agent implement and validate the change.
4. Create a branch, commit the generated changes, and open a GitHub pull request.
5. Return the PR URL and a concise summary back into meeting chat.

## Access and security requirements
- Define the repository, GitHub App/token scopes, and agent-provider credentials required for this flow.
- Ensure the implementation has the needed access for the referenced `5.6-sol` target/repository before attempting changes.
- Require explicit participant review/approval before any external write operation, including branch pushes and PR creation.
- Do not expose credentials or sensitive repository content in chat.

## Acceptance criteria
- A participant can initiate an AI-assisted PR request from Conclave meeting chat.
- The system can run one supported agent harness end-to-end; alternative harnesses are documented or pluggable.
- The system opens a PR in the authorized target repository with implementation details and validation results.
- Chat receives the PR link and status, including actionable errors when access, agent execution, or PR creation fails.
- External writes are gated by explicit approval and operate only on authorized repositories.

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Requested by **a meeting participant** during a Conclave meeting.

Start at Conclave's meeting-chat entry point and trace how participant requests can reach an authorized repository workspace. Compare the listed agent harness options and define the approval, credential, and repository-access boundaries before implementing one end-to-end path. Done means a validated change produces an authorized GitHub pull request, with its URL, status, and actionable errors returned to chat.

Written by the indexing model from the issue text.