Centralize repository context resolution for repo and pull request workflows

[hex2dec]

Follow-up to #2.

Summary

Move Git-derived repository context logic into a deeper shared module so repo and PR workflows rely on one consistent source of truth.

Problem

Repository inference and branch validation logic is currently split between the repository-context module and PR-specific code. That duplication makes behavior harder to reason about and increases the chance of drift.

Scope

• Extend the repository-context layer to cover current branch resolution.
• Extend the repository-context layer to cover origin remote parsing.
• Extend the repository-context layer to cover upstream tracking validation.
• Extend the repository-context layer to cover origin-only branch checks used by PR creation.
• Refactor repo and PR workflows to use the centralized context APIs.

Non-goals

• Do not add support for complex cross-fork or multi-remote PR creation workflows.
• Do not change the public command surface.
• Do not change JSON output contracts.

Acceptance Criteria

• repo view local-context behavior uses the centralized repository-context module.
• pr create local head resolution and origin/upstream validation use the centralized repository-context module.
• Shared Git error paths remain stable and test-backed.
• Repository-context tests cover HTTPS origin URLs.
• Repository-context tests cover SSH origin URLs.
• Repository-context tests cover missing origin remotes.
• Repository-context tests cover detached HEAD state.
• Repository-context tests cover branches not pushed to origin.
• Repository-context tests cover branches tracking a non-origin remote.
• The PR-specific duplicated Git inspection logic is removed or reduced to thin orchestration.

Notes

This issue is about architectural consolidation, not expanding workflow scope.