feat: plan lifecycle and intents system for V1

[Rylorx]

Summary

Adds plan closing and intents as repo-native, skill-driven features for gsync V1 — no code, no infrastructure, just skill instructions and markdown conventions.

Plan Lifecycle

• Plans use frontmatter status: active|closed with closed_date and closed_reason
• Closed plans are archived to archive/ subdirectory of the activity collection
• Agents scan only the activity root for active plans, not archive/
• Never delete plan files — always close and archive

Intents System

• Lightweight pre-plan coordination signals stored as intent-<slug>.md in intents/ subdirectory
• Lifecycle: open → claimed → closed
• Claimed intents require a note (max 150 chars) signaling someone is actively starting work
• claimed = "do not duplicate without coordinating first"
• Agents check intents for overlap before creating new plans (judgment-based, not algorithmic)

Files Changed

• skills/gsync/SKILL.md — 3 new sections: Plan Lifecycle, Intents, Overlap Check Before Plan Creation
• skills/gsync-onboarding/SKILL.md — Updated activity.md and GSYNC.md starter drafts
• README.md — Mention new features
• PLAN-close-and-intents.md — Full design doc

No config.json schema changes. intents/ and archive/ directories are created at runtime by agents following skill instructions.

Test plan

• Read skills/gsync/SKILL.md as a fresh agent — verify you can close a plan by following the instructions
• Read the Intents section — verify you can create, claim, and close intents
• Read the Overlap Check section — verify plan creation triggers an intent scan
• Read skills/gsync-onboarding/SKILL.md — verify the activity.md draft mentions intents and archive
• Verify the starter GSYNC.md draft mentions plan lifecycle and intents in collection conventions