Epic: collapse launcher/agent sprawl into a minimal, config-driven pattern

[mountainowl]

Goal

Collapse the launcher/agent sprawl into a minimal, config-driven pattern.

Target end state

• One poller — bubo-poller; provider from [scm].provider / BUBO_PROVIDER.
• One agent path — the poller runs [agents].reviewer_command directly from config; Codex is just another configured command (no bubo-codex / codex_runner special-casing). Bring-your-own-CLI: any agent with a CLI works.
• One bin dispatcher — bin/bubo <subcommand> with functions; no wrapper-calling-wrapper.
• Kept behind the abstraction (real difference, not duplication): the gitlab.py / github.py provider modules.

Stages (sequential — they share pyproject[project.scripts], test_deploy_layout, bin/, recipes; not cleanly parallelizable)

• Stage A — drop bubo-gh-poller: refactor(A): drop the convenience bubo-gh-poller entry point (provider via config) #66
• Stage B — unify agent execution: refactor(B): unify agent execution — run reviewer_command from config; remove bubo-codex/codex_runner #67
• Stage C — bin/bubo dispatcher (do LAST): refactor(C): collapse bin/ launchers into a single bin/bubo dispatcher #68

Decisions

• Clean-cut (no deprecated shims) — per "no loose ends in production code."
• Pre-1.0 (major_version_zero): each stage is a ! minor bump with a CHANGELOG migration note.