feat(linkedin-studio): second-brain SB-S0 foundation — brain/ scaffold + profile fold + id/provenance spine [skip-docs]

SB-S0 (Foundation) of the second-brain arc: a new TS package scripts/brain/
(structural copy of specifics-bank) establishing the spine later slices hang on.
TDD throughout (failing test first); fold scope = P1+P2 only per operator decision.

- types.ts: one provenance vocab (human|published|ai-draft) + the six-field
  ProfileFact record + two-layer ProfileDoc.
- id.ts: slugify + mintEntityId (sha256[:12], kind-namespaced, slug-keyed so the
  id is stable across value edits) + normalizeProvenance (throws on unknown). [SC4]
- profile.ts: no-YAML line-grammar parse/serialize (parse∘serialize = identity over
  the whole doc; values may contain ]/|/quotes) [SC2] + foldUserProfile: lossless,
  idempotent, source-absent-aware fold of config/user-profile.template.md. Pinned
  extraction — P1 labeled scalars (group-headers skipped, [placeholder]→empty) + P2
  expertise group; stops at "### Research Tooling" so deferred explainer prose can't
  leak in as fields. Checkbox-prefs (Goals/Tone/MCPs/Assets) deferred (§8). [SC3]
- dataRoot.ts: inline per-package resolver (repo idiom; no new seam → SC6).
- scaffold.ts/cli.ts: idempotent `brain init` — brain/{index,profile,operations}.md
  + journal/ + ingest/{inbox,published} under the data-dir; compare-then-skip, never
  clobbers a user edit. No session-start wiring (SB-S2 owns it). [SC1]

Gate: new BRAIN floor in test-runner.sh (34 tests; trends/specifics/contract floors
unchanged) + anti-erosion floor 74→75. Full gate 90/0/0, tsc --noEmit clean. [SC5/SC6]
No new command/agent/reference → no version bump, no structure-count change.

Refs docs/second-brain/{brief,plan-sb-s0,architecture}.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RigJBiRFNtFZKCz21qNbQ4

scripts/brain/fixtures/user-profile.populated.md

@@ -0,0 +1,59 @@
+# User Profile Configuration (populated test fixture)
+
+> Synthetic, domain-general fixture for SB-S0 fold tests. Mirrors the P1 labeled
+> scalars + P2 expertise group of `config/user-profile.template.md`, with every
+> placeholder replaced by a distinctive filled value. NOT a real person.
+
+---
+
+## PERSONALIZATION SETTINGS
+
+### User Profile Context
+
+**Name:** Jordan Avery
+**Current Role:** Senior Data Engineer
+**Organization:** Northwind Analytics
+**Industry/Domain:** Data infrastructure
+
+**Important Disclaimer:** All articles and posts are written as a private individual. Views expressed are personal and do not represent any employer.
+
+**Core Expertise Areas (5 topics):**
+1. Streaming data pipelines
+2. Data contracts
+3. Cost-aware warehousing
+4. Observability for ETL
+5. Team data literacy
+
+**Target Audience:**
+- **Primary:** Data engineers at scale-ups
+- **Secondary:** Engineering managers
+- **Geographic focus:** Nordics
+
+---
+
+### Voice & Style Profile
+
+**Signature Elements:**
+- **Key frameworks you've developed:** The Contract-First Pipeline
+- **Recurring themes/angles:** Boring tech that ships
+- **Phrases you commonly use:** "ship the boring thing"
+- **Topics to AVOID:** Crypto
+
+**Writing Quirks & Preferences:**
+- **Preferred post length:** Medium 1,200-1,800
+- **Emoji usage:** Minimal 1-2
+- **Question style CTAs:** Sometimes
+- **Use of personal anecdotes:** Always
+- **Technical depth:** Advanced
+
+**Language:** English
+
+---
+
+### Strategic Context
+
+**Current LinkedIn Status:**
+- **Follower count:** 2,400
+- **90-day growth goal:** 4,000
+- **Posting frequency:** 3x week
+- **Optimal posting times:** Tue/Thu 08:00 CET