Clean public repository content

[thrr87]

What changed

• remove internal agent/context files from the public tree
• remove unrelated commit-signing setup from the README
• update the README to describe file-backed history, optional folder sync, and the supported Xcode version accurately
• replace development-tool labels and device-specific test names with neutral project terminology
• remove internal test-process notes from public product documentation

Why

The public repository included development-specific guidance and terminology that does not belong in application documentation or source comments. The README also described the pre-migration storage model and an older toolchain.

Impact

There is no runtime behavior change. This only cleans public content and makes the documentation match the application.

Checks

• swift test --disable-sandbox — 18 tests passed
• git diff --check
• repository scan for internal markers, personal paths, device names, and common secret patterns