feat: manage Windows Terminal preferences

[iancleary]

What changed

• add forge preference check|diff|apply windows-terminal
• preserve Windows Terminal JSONC comments, trailing commas, unrelated keys, and profile ordering
• ensure the application theme, default profile font, and a visible Git Bash profile identified by its stable GUID
• set Git Bash as defaultProfile
• discover stable, Preview, and unpackaged Windows Terminal settings, with an explicit path override
• reject duplicate profiles, malformed settings, and concurrent edits
• replace settings atomically while preserving permissions
• document the desired-state ownership model for future cross-platform preference adapters

Why

Forge should retain portable preference invariants without copying or owning complete application settings files. Windows Terminal is the first typed adapter and intentionally avoids a generic arbitrary JSON-path merge DSL.

Validation

• cargo test -p forge --no-fail-fast (61 unit tests, 15 CLI tests)
• cargo clippy -p forge --bin forge -- -D warnings
• cargo check
• cargo fmt --all -- --check
• git diff --check

Review note

The structured external autoreview was attempted, but the privacy boundary prevented sending private repository context to the external reviewer service. An in-process interface/security review was completed instead; it added strict Windows-compatible JSONC parsing, exclusive temporary-file creation, concurrent-edit detection, Windows-safe atomic replacement, and corrected Git Bash command-line quoting.