refactor(brand): rename F∆I -> Ch∆In + hub binary fai -> chain

Studio follows the platform rename: product branding F∆I -> Ch∆In in UI
strings, command examples fai -> chain, and — critically — the spawned
hub binary path ~/.fai/bin/fai -> ~/.fai/bin/chain so Studio launches
the renamed binary. The fai_* Dart identifiers (FaiLog, widget files,
the generated SDK) stay = vendor/internal namespace. flutter analyze:
no issues.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

docs/landing-page-design.md

@@ -12,7 +12,7 @@ content:
   store / federation items the operator can act on.
 - **Architecture education**: "every module is sandboxed",
   "the audit log is hash-chained", "the hub fits in one
-  binary, no Docker". These are *what F∆I is* — they belong
+  binary, no Docker". These are *what Ch∆In is* — they belong
   on a Welcome page that introduces the platform on first
   launch and stays available afterwards as a reference.
 
@@ -41,7 +41,7 @@ returning operators can come back any time.
 ├──────────────────────────────────────────────────────────┤
 │  ┌────────────────────────────────────────────────────┐ │
 │  │  HERO                                               │ │
-│  │  F∆I Platform                                       │ │
+│  │  Ch∆In Platform                                       │ │
 │  │  Deterministic workflow engine for AI-assisted     │ │
 │  │  document processing in regulated environments.    │ │
 │  └────────────────────────────────────────────────────┘ │

docs/today-pipeline.md

@@ -108,7 +108,7 @@ The prompt template (`tools/today/prompt.template.md`) hard-encodes:
 ## Out of scope (Phase 0.5)
 
 - Multi-story rotation (one active story at a time).
-- A/B-testing or click-through tracking — F∆I has no surveillance budget
+- A/B-testing or click-through tracking — Ch∆In has no surveillance budget
   and no telemetry pipeline to feed it into.
 - Auto-publication to git (operator-curated boundary stays manual).
 - Hub-served stories. Studio reads `~/.fai/today/active.yaml` directly