feat: add github-issues task backend

[fyodoriv]

Why this is needed / Motivation

VISION.md G5 ("File-first, pluggable backends") establishes that TASKS.md is the default, but the spec/parser/CLI/MCP MAY target an alternative backend. This PR delivers the first such backend — GitHub Issues — so a team already living in an issue tracker can use the tasks.md workflow (pick/list/create/claim/complete) against Issues without migrating to a file. Downstream consumers (agentbrew, dotfiles) delegate to this CLI rather than each hand-rolling a gh integration.

What changed

• packages/cli/src/backend/ — a TaskBackend port (types.ts) with two adapters:
  • github-issues.ts — reads/writes via gh (open issues by marker label; priority/P0..P3 labels; assignee = claim; close = complete). repo token scope only — no GitHub Project required.
  • tasks-md.ts — the default, reusing the parser's deterministic pickBestTask/loadAllTasks for reads.
  • config.ts — resolves the backend from .tasksmd.json (default tasks-md); --backend flag overrides.
• pick / list are now backend-aware; new create / claim / complete commands work on either backend.
• spec.md — new "## Task backends" section documenting selection, the Issues mapping, and the commands.

tasks-md stays the default, so existing repos are unaffected (G4 preserved — the spec, not the storage, is the portable layer).

Vision trace

• Vision goal: G5 "File-first, pluggable backends" — this is the first backend adapter behind the spec surface; TASKS.md remains the default.
• User story: N/A — foundational tooling; driven by downstream consumers (agentbrew/dotfiles/oncall-hub) adopting an Issues backend.
• Competitor prior art: N/A — extends tasks.md's own backend model (builds on the existing sync module's gh integration).

Test plan

• npm run build clean; npm test green across all workspaces (cli 209, parser 84, mcp 111, lint 62).
• 26 new backend tests (backend/*.test.ts): github-issues mapping/next/create/claim/complete (mocked gh), config resolution + precedence, tasks-md read/create/complete, priority sort.
• Smoke-tested the built CLI end-to-end against a temp TASKS.md (pick/create/list).

Follow-up

• MCP tools backend-awareness + workspace-mode cross-backend aggregation remain (tracked by the github-issues-backend TASKS.md entry).

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🤖 Written by an agent, not Fyodor. Ping me if this looks off.