feat: omnifunc completion for compose buffer metadata

[barrettruth]

Summary

Adds context-aware completion for metadata fields in compose buffers (PR create, PR edit, issue create). Closes #45.

When the cursor is on a Labels:, Assignees:, Reviewers:, or Milestone: line inside the compose comment block, completion candidates are fetched from the forge API and offered via Neovim's built-in omnifunc.

How it works

• lua/forge/completion.lua — new module implementing the omnifunc

  • Detects which metadata field the cursor is on via line pattern matching
  • Fetches candidates from f:completion_cmd(field) (newline-delimited output)
  • Caches results per-forge per-field with a 5-minute TTL
  • Filters by typed prefix (case-insensitive)

• completion_cmd(field) — new backend method on all 3 forges:

  Field	GitHub	GitLab	Codeberg
  labels	gh label list --json name --jq .[].name	glab label list	tea api /repos/:owner/:repo/labels
  assignees/reviewers	gh api repos/{nwo}/collaborators --jq .[].login	glab api projects/:id/members/all	tea api /repos/:owner/:repo/collaborators
  milestone	gh api repos/{nwo}/milestones --jq .[].title	glab api projects/:id/milestones?state=active	tea api /repos/:owner/:repo/milestones?state=open

• compose.lua — sets omnifunc on all compose buffers via create_compose_buf()

User experience

• With blink.cmp or nvim-cmp: completions appear automatically (both plugins pick up omnifunc sources)
• Without a completion plugin: trigger with Ctrl-X Ctrl-O (standard Vim omni-completion)
• Completions only activate on metadata lines inside the <!-- --> comment block — no interference with title/body editing

Files changed

• lua/forge/completion.lua — new (omnifunc + cache)
• lua/forge/compose.lua — +1 line (set omnifunc)
• lua/forge/github.lua — +completion_cmd()
• lua/forge/gitlab.lua — +completion_cmd()
• lua/forge/codeberg.lua — +completion_cmd()
• lua/forge/config.lua — +type annotation

Test plan

• All 158 existing tests pass
• StyLua formatting clean
• Manual: open compose buffer, cursor on Labels line, Ctrl-X Ctrl-O shows repo labels
• Manual: cursor on Assignees line shows collaborators
• Manual: cursor on Milestone line shows milestones
• Manual: completions are cached (second trigger is instant)
• Manual: works with blink.cmp auto-completion