feat(project): author the planning-suite mission for taskfiles (#262)

[nolte]

Summary

Authors the project/ planning suite for taskfiles so it ships a valid project/mission.md, closing the mission-authoring half of the portfolio backfill tracked in nolte/claude-shared#262 (taskfiles is one of the two prioritised W2 repos). The manifest (project/portfolio.yml) already exists; this PR adds the missing planning artefacts the mission spec (spec/project/mission/) requires as hard preconditions.

taskfiles is a mature, portfolio-wide configuration source whose reusable-taskfile-collection capability was already status: active before the planning suite existed, so the MVP is modelled retroactively (roadmap item and sprint/feature recorded done/closed, mvp_status: achieved). This mirrors the merged gh-plumbing pilot (nolte/gh-plumbing#383) and nolte/kamerplanter's retroactive sprint 0001.

Changes

• project/goals.md — Vision (from the README intro) + four Outcomes (O-1..O-4) from the shipped capability and its audiences.
• project/roadmap.md — R-1 (reusable Taskfile include collection), mvp: true, detail: fine, target_sprint: 1, status: done.
• project/sprints/0001-shared-task-baseline.md — retroactive sprint 1 (closed) with the consumer-facing value_statement.
• project/features/reusable-taskfile-adoption.md — F-1, the mission-verifying feature (verifies_sprint_value: acceptance-1).
• project/mission.md — SMART mission with the eight required frontmatter fields, the four required sections, a per-audience paragraph for each of the four audiences, and verifies_via: F-1:acceptance-1.

All content is grounded in existing repo artefacts (README.md, project/portfolio.yml, AUDIENCES.md) — no invented audiences or missions, per the nolte/claude-shared#262 constraint.

Linked issues

Part of nolte/claude-shared#262 (mission-authoring backfill).

Testing

Structural conformance authored against spec/project/mission/, roadmap/, sprint/, and feature/; frontmatter field order, section order, audience↔paragraph bidirectionality, and verifies_via → feature/acceptance resolution checked by hand. Vale-clean against the repo's .vale.ini (Microsoft + nolte-styles).

Risk / rollout notes

Additive project/ documentation only; no module, config, or published-surface change. Trivially reversible.