chore(release): bump version to 2.1.6 (build 131)

Version: CURRENT_PROJECT_VERSION 130 -> 131 (project.yml, AGENTS.md, READMEs,
docs/llms.txt).

fix(flow): host-app identification now compiles in every build configuration
(Debug, TestFlight, Release). The arbiter probe and HostAppIdentity were
DEBUG-only, so TestFlight/App Store builds never knew the host bundle id and
could not auto-return after a startflow handoff. Debug-only logging and the
Home test card are unchanged.

Also includes the pending work in the tree:
- Home: single account/setup card surface (HomeAccountSurfacePolicy/Views)
- Wide screens: voice key row and centred mic placement, iPhone Duo layout
- Keyboard: clipboard skill ranking and AI skill layout updates
- Debug: account session handoff to the Simulator (+ import script)
- Docs: portfolio assets, privacy policy URL, site/README refresh

docs/llms.txt

@@ -2,7 +2,7 @@
 
 > OSGKeyboard is an agentic keyboard and AI voice keyboard (Chinese: AI 语音输入法 / 嘴替输入法) for iPhone, iPad, and Mac. The iOS keyboard combines on-device dictation, a unified AI assistant, a learned personal style that sounds like you, agentic clipboard skills, Chinese and English typing, and optional managed credits. On Mac, hold Option for global dictation. It is source available, not open source.
 
-Current repository version: 2.1.5 (build 125). Later work is listed under CHANGELOG [Unreleased].
+Current repository version: 2.1.6 (build 131). Later work is listed under CHANGELOG [Unreleased].
 
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