docs: add github-issues-backend task to queue

[fyodoriv]

Why this is needed / Motivation

Downstream consumers of the tasks.md spec (agentbrew, minsky, dotfiles, and the oncall-hub repos) are adopting GitHub Issues + Projects as a task backend so individual repos can retire their TASKS.md file. The canonical tasks.md tooling (spec + parser + CLI + MCP) is the load-bearing layer they build on, so rather than each consumer hand-rolling its own gh integration, tasks.md can offer an opt-in Issues backend behind the same spec surface.

What changed

• VISION.md — adds goal G5 "File-first, pluggable backends": TASKS.md (local markdown) stays the default/canonical backend; the spec, parser, CLI, and MCP MAY target an explicit, off-by-default alternative backend (e.g. GitHub Issues/Projects) so a team already in a tracker can adopt the workflow without migrating. Reframes the "GitHub Issues" bullet from a flat rejection to "as your only queue" + the opt-in backend path. Explicitly preserves G4 — the spec, not the storage, is the portable layer.
• TASKS.md — queues the github-issues-backend task (the first G5 adapter), composing with the existing workspace-mode-nested-repos task.

Vision trace

• Vision goal: G5 "File-first, pluggable backends" (added in this PR) — the github-issues-backend task is the first backend adapter behind the spec surface; TASKS.md remains the default, preserving G4.
• User story: N/A — spec/tooling planning entry; the need is driven by downstream consumers (agentbrew / minsky / dotfiles / oncall-hub) adopting an Issues backend.
• Competitor prior art: N/A — this extends tasks.md's own backend model rather than matching a competitor product.

Test plan

• npx @tasks-md/lint TASKS.md → 0 errors; diff is additions-only for TASKS.md (+20/−0) and a 5-line VISION.md amendment.
• pr-vision-trace satisfied by the block above; lint / test / commands-drift unaffected (no parser/CLI/spec code change).

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