🌱 design: forge-neutral enumeration engine (ADR)

[Danathar]

Summary

Every hive work cycle starts from enumeration: EnumerateActionable in
pkg/github lists a repo's open issues and PRs and — in the same pass —
applies the policy that makes hive safe to run: hold and exempt labels,
project.issue_filter, SLA tracking, tracker detection, the standing-meta
skip (#6158), clustering. Because that choke point is GitHub-only, a hive
configured for any other forge enumerates nothing, which is why the forge
adapters sit "wired but unreachable" (forge-app-setup.md). The roadmap names
neutralizing this as the next step for the forge abstraction; this task
designs it before anyone moves code.

Part of #6167 (Gitea forge support series).

What the ADR must decide

• Where lifted policy lives. Working proposal: a new pkg/actionable
  consuming forge.Forge + policy config and producing ActionableResult;
  alternative is housing it under pkg/forge. Maintainer preference asked in
  🌱 design: Gitea forge support — wave plan from adapter-only to working agents #6167.
• The exact field gap between forge.Issue/forge.ChangeRequest and
  what fetchIssues/fetchPRs consume (timestamps, author, body, draft
  status, assignees, head ref, …) — enumerated exhaustively, since it becomes
  the wave-1 data-type PR's checklist.
• The bulk-list shape the roadmap asks for, so an N-repo hive does not
  pay N× enumeration, with a pagination budget.
• The compatibility proof. Golden tests running the old GitHub path and
  the new engine on identical fixtures, requiring byte-identical
  ActionableResult — the merge gate for the later extraction PR, designed
  now so the extraction cannot drift.
• Scope statement: one forge per hive (project.forge stays a single
  global selector); mixed-forge hives explicitly deferred.

Constraints

• GitHub behavior must be byte-identical through the whole series — the
  policy in fetchIssues is documented as the reason label gates cannot be
  bypassed by a re-listing agent, and none of its semantics may shift as a
  side effect of moving.
• The seam should follow the shape ✨ pkg/forge: move scheduler operations behind the abstraction (roadmap 'Next', untracked since #2812 closed) #5259 already established for escalation
  writes: GitHub keeps its concrete client; only the static type at the call
  site changes.

Deliverable / definition of done

An ADR under src/docs/adr/ (PR): the chosen home, the field-gap table, the
bulk-list interface sketch, the golden-test plan, and the scope statement.
Docs only; no behavior change.

Model / effort

Planned execution: Claude Fable 5 at max effort — the field-gap
enumeration sweeps go to low-effort subagents; the design and the ADR text
stay at max.