docs(workstation): add world-class SourceOS and Mac-on-Linux gap plan

[Copilot]

Adds docs/workstation/world-class-sourceos-parity-plan.md to consolidate the Mac-on-Linux / world-class SourceOS gap analysis into a single actionable spec. Two corrections are integrated: (1) we are near a credible Mac-like GNOME workstation v0, not macOS parity; (2) sourceos-shell already exists — the next move is populating its PDF-first runtime lane, not repo creation.

Document structure

• Tier definitions — explicit separation of Mac-like GNOME workstation v0 (in progress), macOS parity (non-goal), and world-class OS (not yet attempted)
• Landed work — tables of implemented/validation-backed surfaces in source-os and sourceos-spec
• Gap catalogue — nine prioritised gaps: shell runtime alignment, backup/recovery, security/trust envelope, continuity equivalents, contract canon, PDF/document runtime, app/runtime trust, accessibility/localization, evidence/reporting
• Parity scorecard — 18-row maturity table across all three tiers; overall verdict: Tier 1 in progress, Tier 2 and Tier 3 not yet attempted
• Repo ownership — boundary table for source-os, sourceos-spec, sourceos-shell, and SocioProphet agent/control-plane repos
• Execution plan — nine work packets (WP-1 through WP-9) in priority order, each with a shell-testable acceptance gate

Validation

test -s docs/workstation/world-class-sourceos-parity-plan.md           # ✅
grep -F "sourceos-shell already exists" ...parity-plan.md              # ✅
grep -F "Mac-like GNOME workstation v0"  ...parity-plan.md             # ✅
grep -F "world-class OS"                 ...parity-plan.md             # ✅

No scripts, workflows, or package manifests modified.