refactor!: unify forge:// bufnames to host/slug/resource scheme

[barrettruth]

Problem

forge.nvim had six ad-hoc bufname formats: forge://pr/new, forge://pr/{num}/edit, forge://issue/new, forge://issue/{num}/edit, forge://log/{title}, forge://ci/{title}. The first four collide across repos (:Forge pr edit 42 in repo A vs repo=upstream pulls both to forge://pr/42/edit). The last two collide on their default 'ci' / 'summary' titles when the same CI run is opened twice — the pcall around nvim_buf_set_name was silently swallowing the failure, producing unnamed duplicate buffers. Compose/log/ci buffer naming had zero scoping.

Solution

Single scheme:

forge://{host}/{slug}/{resource}/{id-or-action}[/{sub}]

• scope.bufpath(scope) new helper returning "{host}/{slug}" or nil
• compose.lua gains a local resolve_bufpath(ref, forge_name) that tries the explicit scope first and falls back to forge.current_scope(forge_name) via origin remote. All 4 compose open sites use it; pcall dropped from nvim_buf_set_name. When no scope can be resolved (not in a repo, no detectable remote) compose refuses to open with a warning instead of creating a nameless buffer.
• log.lua LogOpts / SummaryOpts drop title and gain scope + run_id. New helpers log_bufname() / summary_bufname() produce forge://{host}/{slug}/ci/{run_id}[/log|/job/{job_id}]. Both M.open and M.open_summary now look up an existing buffer by name before creating a new one, fixing the duplicate-unnamed-buffer bug.
• ops.lua 5 callsites (3× log.open, 2× log.open_summary) thread scope = run_ref and run_id = run.id through the opts tables; drop the old title = ... composition.
• pickers.lua 1 callsite in the PR checks picker threads scope = check_ref and run_id = run_id.

Concrete examples:

forge://github.com/barrettruth/forge.nvim/pr/42/edit
forge://github.com/barrettruth/forge.nvim/ci/987654321
forge://github.com/barrettruth/forge.nvim/ci/987654321/job/5544
forge://gitlab.com/group/subgroup/project/pr/42/edit
forge://ghe.company.com/team/app/pr/new

8 spec files updated: mocks gain current_scope, opts assertions use scope/run_id instead of title, literal bufnames updated to the new format. doc/forge.nvim.txt compose-section bufname references updated; PROSPECTIVE_CHANGES.md line 179's revisit TODO is now addressed.

Breaking: LogOpts.title and SummaryOpts.title are gone; both now require scope + run_id (scope is optional at the type level to tolerate test stubs with partial context, but production paths all supply it). Existing bufname literals anyone scripted against no longer match.

scripts/ci.sh green: stylua / selene / prettier / nix fmt / lua-language-server / vimdoc-language-server / busted (443/0).