feat: edit existing PR via compose buffer

[barrettruth]

Summary

Adds the ability to edit an existing PR through the same compose buffer UX used for creation. Closes #21.

Entry points

• :Forge pr edit {num} — direct command to edit a specific PR
• <c-e> in PR picker — new keybinding opens edit buffer for the selected PR
• Auto-redirect — :Forge pr create on a branch with an existing PR now opens the edit buffer instead of bailing out with a warning

Keybinding changes

• edit = <c-e> (previously manage)
• manage = <c-m> (new binding)

Backend methods added (all 3 forges)

Method	GitHub	GitLab	Codeberg
fetch_pr_details_cmd(num)	gh pr view --json ...	glab mr view --output json	tea api /repos/:owner/:repo/pulls/{num}
update_pr_cmd(...)	gh pr edit with --add-* flags	glab mr update	tea pr edit (title/body only)
parse_pr_details(json)	labels, assignees, reviewRequests, milestone	labels, assignees, reviewers, milestone	labels, assignees, milestone

Compose buffer for editing

• open_pr_edit() opens a compose buffer pre-filled with the PR's current title, body, and all metadata
• Buffer name: forge://pr/{num}/edit
• Comment block says "Editing Pull Request #N via github"
• On :w, calls update_pr_cmd() instead of create_pr_cmd()
• Draft toggle handled separately via draft_toggle_cmd() when draft state changes

Files changed

• lua/forge/github.lua — +3 methods
• lua/forge/gitlab.lua — +3 methods
• lua/forge/codeberg.lua — +3 methods
• lua/forge/compose.lua — update_pr() + open_pr_edit()
• lua/forge/init.lua — edit_pr() + auto-redirect in create_pr()
• lua/forge/pickers.lua — edit action in PR picker
• lua/forge/config.lua — type annotations + keybinding defaults
• plugin/forge.lua — :Forge pr edit dispatch + tab completion

Test plan

• All 158 existing tests pass
• StyLua formatting clean
• Manual: :Forge pr edit {num} fetches and opens compose buffer
• Manual: <c-e> in PR picker opens edit buffer
• Manual: :Forge pr create on branch with existing PR auto-redirects to edit
• Manual: Editing title/body/metadata and writing updates the PR
• Manual: Changing draft state toggles draft on save