docs(showcase): add PRP-39 and PRP-40 planning artifacts

[w7-mgfcode]

Goal

Land the planning artifacts for the /showcase rich-demo upgrade epic as a docs-only PR so subsequent implementation PRs (PRP-39 + PRP-40 execution) carry clean code diffs only.

Scope

11 untracked files generated this session, none touching app/ or frontend/src/:

Umbrella + index + walkthrough (3)

• PRPs/INITIAL/INITIAL-showcase-rich-demo-control-center.md
• PRPs/INITIAL/INITIAL-showcase-rich-demo-index.md
• docs/user-guide/showcase-walkthrough.md

Per-PRP INITIALs (4)

• PRPs/INITIAL/INITIAL-showcase-38-data-modeling-lifecycle.md
• PRPs/INITIAL/INITIAL-showcase-39-decision-portfolio-lifecycle.md
• PRPs/INITIAL/INITIAL-showcase-40-planning-knowledge-lifecycle.md
• PRPs/INITIAL/INITIAL-showcase-41-agent-ops-polish.md

Generated PRPs + Task-1 contract-probe reports (4)

• PRPs/PRP-39-showcase-decision-portfolio-lifecycle.md (1487 LOC)
• PRPs/PRP-40-showcase-planning-knowledge-lifecycle.md (1451 LOC)
• PRPs/ai_docs/prp-39-contract-probe-report.md (318 LOC)
• PRPs/ai_docs/prp-40-contract-probe-report.md (343 LOC)

Already tracked (NOT duplicated by this PR)

• PRPs/PRP-38-showcase-data-modeling-lifecycle.md — landed in feat(api,ui): showcase pipeline — richer data + V1/V2 modeling foundation (PRP-38) #309 / PR feat(api,ui): showcase pipeline richer data and v2 foundation (#309) #310 / 9c4bb91.
• PRPs/ai_docs/prp-38-contract-probe-report.md — same.

PRP dependency graph

PRP-38 (merged on dev @ 3e771c9)
   │
   ├── PRP-39  (decision + portfolio lifecycle — ready to execute)
   │
   └── PRP-40  (planning + knowledge lifecycle — ready to execute, parallel to PRP-39)
           │
           └── PRP-41  (agent + ops + polish — gated on BOTH PRP-39 + PRP-40)

PRP-39 and PRP-40 are sibling slices that can be authored AND executed in parallel — they share no step names, no test names, no file edits beyond _phase_table() + PHASE_DEFS.ts, and both use RELATIVE phase anchors so the second-to-merge rebases cleanly.

Commit subject

docs(docs): add rich showcase planning artifacts (#<this issue>) — docs(showcase) is not in the commit-format scope allow-list; docs(docs) matches the existing precedent (#283 #296).

Out of scope

• Any change under app/ or frontend/src/.
• Execution of PRP-39 or PRP-40 (separate follow-up PRs).
• Changes to PRP-41 (it stays gated on PRP-39 + PRP-40 merging first).

[w7-mgfcode]

Merged on dev via PR #314 — PRP-39 / PRP-40 planning artifacts are in the repo. Closing as hygiene.