feat(config): support draft GitHub pull requests

[farsyed]

This change adds opt-in support for draft GitHub pull requests.

Summary:

• Adds opt-in draft_pr: true at global and repository configuration scopes.
• Repository configuration overrides the global configuration.
• The default is false.
• New GitHub PRs are created with --draft, including PRs from forks.
• Existing PR updates and non-GitHub providers are unchanged.

Tests:

• go test ./internal/config ./internal/scm/github ./internal/pipeline/steps
• make lint
• git diff --check

[kunchenguid]

Speaking as Kun's firstmate: I reviewed the diff (config merge, GitHub CreatePR --draft, tests, docs). This is opt-in (draft_pr defaults false; repo overrides global; existing PR updates never change readiness). I approved the first-time fork workflow runs so CI can start.

This PR is still a draft, so it will not be merged from this pass. After it is marked ready and CI is green — including the no-mistakes raise check — it can be reconsidered. Please raise it through the no-mistakes gate if that check fails.

[kunchenguid]

Speaking as Kun's firstmate: Re-read the diff (config merge, GitHub CreatePR --draft, tests, docs). Still opt-in: draft_pr defaults false, repo overrides global, updates never change readiness. Hosted tests/e2e/check are green. No additional fork workflow runs were waiting for approval.

This PR is still a draft, and PR must be raised via no-mistakes is still red, so it will not be merged. Please mark it ready when you intend it to ship, and raise it through the no-mistakes gate.

[kunchenguid]

Speaking as Kun's firstmate: this PR covers issue #761 (opt-in GitHub draft PRs; default false). Not labeling #761 ready-for-pr.

Re-read the diff: draft_pr on global + repo config, repo overrides global, github.Host.WithDraftPR adds --draft only on pr create (including fork heads), updates leave readiness unchanged, GitLab/Azure DevOps untouched. Tests cover merge defaults, create argv, and "no --draft on edit".

Still a draft, and PR must be raised via no-mistakes is still red, so it will not be merged from this pass. Please mark ready when you intend it to ship, raise it through the no-mistakes gate, and consider adding Addresses #761 in the body so the issue is linked. Optional follow-up (not blocking for the standing-rule case): axi run --draft for per-change draft-ness; config already solves the team-wide rule.

VISION: R1/R3/R7 align (opt-in, default false, local config). No captain flag.