feat: issues-only SDD backend for loadout spec and the wizard

[machado144]

Refs #25 — spec 011 is pending review; this is a draft until it is approved.

What

Adds a third SDD backend, issues, where each spec and its plan live in one GitHub issue and nothing is written to the repo. The flow matches the hand-edited spec-af/plan-af/handoff-af/resume-af/sdd-workflow-af/sdd store files, so re-running the wizard with sdd.backend: issues regenerates them instead of reverting to docs/specs.

loadout spec --backend issues (or LOADOUT_SDD_BACKEND=issues)

$ loadout spec init "drift-safe sync" --backend issues
Opened spec issue #20: spec(010): drift-safe sync (acme/demo)

$ loadout spec list --backend issues
SPEC                 STEPS  BACKEND
011-issues-backend   1/3    issue #25

$ loadout spec check '#25' 3 --backend issues   # ticks "- [ ] **3. …**" between the plan markers

• init: numbers after both existing spec(NNN): issues and local spec files. It creates the spec label if missing and uses the template body with the plan placeholder (--style full|light).
• list/status: read issues by label. Steps are the checkboxes between <!-- plan:start -->/<!-- plan:end -->, addressed by their **N. number or an sdd: marker. Code fences and text outside the markers are ignored.
• check/uncheck: re-fetch the body and change only that checkbox character.
• sync: refused with a "not applicable" message, since the issue is the only copy.

Wizard

sdd.backend gains issues. With it, /spec, /plan, /handoff, /resume, the workflow skill, the sdd instruction and context-handoff describe the issues-only flow. With your custom names, the generated files differ from your current store copies only in three added hints:

• /spec-af: a loadout spec init … --backend issues shortcut;
• /plan-af and the skill: loadout spec status/check … --backend issues as the fetch-flip-write helper;
• context-handoff: its location and lifecycle lines now say "spec issue comment / untracked local file". Your store copy still says .agents/context/handoff.md, "safe to commit".

local and github output is unchanged; tests compare it byte-for-byte for handoff/resume.

Tests

• internal/sdd/issues_test.go: plan-section parsing (number vs marker IDs, fences, outside-marker lines, CRLF), single-byte toggle, valid-ID errors, duplicate numbers, #N resolve, slugify, and backend: issues in sdd.yaml.
• internal/wizard/content_issues_test.go: key phrases per artifact, no .plan.md/sdd.yaml in issues mode, light style, and the other backends unaffected.
• integration/spec_issues_test.go (real binary + stub gh tracker): init numbering, label and body, nothing written to the repo, duplicate refused; list/check/uncheck/status with an edit made in the GitHub UI between commands preserved, every issue edit immediately preceded by issue view; sync refused; env var selection; and wizard --answers installs the issues-only artifacts.
• go test -race ./..., -tags integration, golangci-lint (0 issues).

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[github-actions]

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Current title: feat: issues-only SDD backend for loadout spec and the wizard