Agent plans: status taxonomy and assembly inspection note

[trailcode]

Summary

• Standardize agents/plans YAML statuses (planning / partial / done / deferred) and index them in the plans README.
• Add deferred post-v1 design note for Assembly inspection mode (depends on hierarchy phase 3 placement).
• Refresh per-plan status fields to match shipped vs deferred work.

Test plan

• Skim agents/plans/README.md status table for consistency with each plan front matter
• Confirm assembly-inspection-mode.md correctly defers until phase 3 / Shape List hierarchy phase 3: parent transforms and advanced assembly model #214
• No product/runtime code changes — docs-only merge

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Note

Low Risk
Documentation and agent-plan metadata only; no runtime or product code changes.

Overview
Standardizes how feature plans are labeled under agents/plans: YAML status is now one of planning, partial, done, or deferred, with meanings and update guidance documented in the plans README. The index table gains a Status column, and the sketch-from-shape discovery diagram annotates each step with its current status.

Realigns existing plan front matter from mixed synonyms (implemented, implemented-v0, planned, planning) to that vocabulary—for example cross-section and configurable hotkeys → done, shape hierarchy phase 3 → deferred, shape origin → partial, WASM items → planning.

Adds assembly-inspection-mode.md as a deferred post-v1 design note: a future third idle context (sketch / part / assembly) that should wait on phase 3 relative placement (#214), reuse one Move/Rotate/Duplicate family with semantics by selection kind, and avoid shipping while transforms still bake BREP.

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