Add --headless mode for CI/CD and GitHub Actions usage

[snipcodeit]

Description

Add a --headless flag to the MGW CLI for non-interactive CI/CD usage.

Scope (consolidated from closed issues #126, #127, #128)

Contract design (was #126):

• Define which commands support --headless
• How prompts are handled in headless mode (default to safe action, no stdin reads)
• Structured JSON progress output format for CI consumers
• Exit code contract: 0 on success, non-zero on any pipeline failure
• Write spec in docs/HEADLESS.md

Implementation (was #127):

• Add --headless to mgw:run and mgw:issue
• Skip confirmation prompts, use defaults
• Emit structured JSON progress to stdout per pipeline event
• Exit codes 0/non-zero

GitHub Actions templates (was #128):

• Add .github/workflows/mgw-auto-triage.yml (on: issue opened) as opt-in template
• Add .github/workflows/mgw-milestone-exec.yml (workflow_dispatch) as opt-in template
• Document how users copy these to their own repos

Notes

Depends on multi-provider architecture being stable (shipped in v5). Each provider will need headless behavior defined separately. The CI workflow templates depend on headless mode working correctly.

Acceptance Criteria

• --headless flag accepted by mgw run and mgw issue
• No stdin reads in headless mode
• JSON progress events emitted to stdout
• Exit code reflects pipeline success/failure
• docs/HEADLESS.md spec written
• Two GitHub Actions workflow templates provided as opt-in examples

Prior Issues

Consolidated from: #126, #127, #128