feat(mcp): make task tools github-issues-backend-aware

[fyodoriv]

Why this is needed

The single-repo github-issues backend shipped in #88 (CLI pick/list/create/claim/complete + a backend-agnostic Task). The MCP server — how most agents read/write tasks — still operated only on TASKS.md, so on an issue-backed repo the MCP tools and the CLI disagreed about where tasks live. This makes the five MCP task tools backend-aware so both surfaces behave identically.

What changed

• packages/mcp/src/backend.ts (new) — resolveBackend(dir) (reads .tasksmd.json at the git root, default tasks-md), runTasksCli(args, cwd) (delegates to the tasks binary; local node_modules/.bin/tasks → npx -y @tasks-md/cli), and pure buildListArgs/buildPickArgs argv builders.
• packages/mcp/src/index.ts — list_tasks/pick_task/claim_task/complete_task delegate to the tasks CLI when the repo is github-issues (claim-on-pick preserved); unclaim_task returns a clear "unsupported on github-issues" message; the tasks-md path is untouched.
• packages/mcp/src/index.test.ts (new) — resolveBackend/runTasksCli tests + argv-contract tests that pin the delegated flag surface (and assert it never emits --repo/--id, which the CLI's list/pick reject).
• TASKS.md — scoped github-issues-backend down to its sole remaining piece (cross-backend aggregation) and blocked it on workspace-mode-nested-repos.

Delegation keeps the backend single-sourced in the CLI rather than re-implementing a gh client in the MCP package (GET-don't-implement).

Vision trace

• Vision goal: tasks.md "Prefer small, obvious APIs … merge behavior instead of adding another public command" — MCP reuses the CLI backend rather than forking a second implementation.
• User story: N/A — agent-facing MCP parity with the CLI backend; driven by the downstream github-issues rollout (agentbrew/dotfiles).
• Competitor prior art: N/A — internal task-backend delegation.

Test plan

• npm run build clean; npm test green across all workspaces (CLI 209, lint 62, MCP 125, parser 84).
• Validated the delegated flags against the real built CLI: tasks list --json --priority P1 --tag x --unclaimed --unblocked and tasks pick --json --tags a,b both exit 0 with no stderr (commander accepts every flag the builders emit).
• npx -y @tasks-md/lint TASKS.md → 0 errors.

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