Add a Workfront Planning solution-architect skill under a new workfront plugin

### Problem
The marketplace has no coverage for Adobe Workfront Planning (WFP, aka Maestro), the strategic-planning product inside Workfront. Internal engineers, solution architects, and account teams doing Planning work have no guided, discoverable path and repeatedly rediscover the same non-obvious knowledge, specifically:

* Whether a customer's limit problem (e.g. the ~500 connected-records cap) is actually an architecture problem in disguise
* How object limits scale across the Select / Prime / Ultimate tiers
* Designing workspaces, record types, and connected records without hitting hierarchy/connection caps
* Debugging Planning formula and connection failures
* Reconciling public docs (UI surface) against the Planning MCP/API (automation surface) when they disagree
* Framing internal P95 / latency telemetry correctly when sharing with customer-facing colleagues (telemetry, not an SLA)
* Building workspaces agentically via the Planning MCP

### Proposed Solution
Add a new `workfront` plugin to house Workfront Planning skills, starting with a single solution-architect skill:

* `workfront` — new plugin (`.claude-plugin` / `.cursor-plugin` manifests, plugin README, marketplace entry)
* `wf-planning-solution-architect` — acts as an experienced WFP solution architect: workspace and record-type design, connected records, formula debugging, tier and object limits, performance-telemetry framing, and agentic workspace builds via the Planning MCP/API

Includes two authoritative reference layers (`references/raw/` mirroring public docs, `references/mcp/` documenting the MCP/API surface), a public-vs-MCP discrepancy guide, a workspace-build playbook, a fully synthetic best-practice workspace export, and evaluations covering the connection-cap, tier-limits, P95-telemetry, and docs-vs-MCP scenarios.

### Value
Gives agents a guided, discoverable entry point for Workfront Planning solution-architecture work and surfaces the field's recurring gotchas — the connection-cap-as-architecture-signal pattern, tier-dependent limits, and the telemetry-vs-SLA distinction — proactively, instead of requiring per-customer rediscovery.

Start with the proposed workfront plugin manifests, plugin README, and marketplace entry, then map the wf-planning-solution-architect skill across references/raw/, references/mcp/, the discrepancy guide, workspace-build playbook, synthetic export, and evaluations. Done means the plugin and skill cover the listed Planning scenarios and include all specified reference and evaluation materials.

Written by the indexing model from the issue text.