docs(setup): document interactive tool picker (#495 Slice 4) - #508

Summary

Documents the completed cross-platform interactive tool picker from #495 after PRs #505 and #506. The README now owns the public invocation and behavior contract, while ARCHITECTURE.md owns implementation boundaries and ordered-selection guarantees.

Changes

• Document automatic and explicit picker invocation on Bash and PowerShell.
• Document default and opt-in states, ordering, controls, confirmation, cancellation, and empty selection.
• Document Bash 4.2+ arrow mode, Bash 3.2 numbered fallback, and the PowerShell 5.1-safe ASCII menu.
• Document CI/headless suppression, the explicit non-interactive escape hatch, and flag precedence.
• Update architecture ownership, helper loading, current default dependency order, and picker CI coverage.
• Keep hidden test seams out of the public documentation.

Test plan

• C:\Program Files\Git\bin\bash.exe tests/test_setup_flags.sh -- 50 passed
• C:\Program Files\Git\bin\bash.exe tests/test_tui_bash.sh -- 32 passed
• powershell -NoProfile -ExecutionPolicy Bypass -File tests\test_setup_flags_pwsh.ps1 -- 50 passed
• powershell -NoProfile -ExecutionPolicy Bypass -File tests\test_tui_pwsh.ps1 -- 10 passed
• C:\Program Files\Git\bin\bash.exe setup.sh --help -- passed
• powershell -NoProfile -ExecutionPolicy Bypass -File setup.ps1 -Help -- passed
• python scripts\lib\ascii-sweep.py --dry-run -- 16 Markdown files scanned, 0 edits
• git diff --check -- passed

Related

Closes #495

Hygiene checklist

• ASCII-only in all .ps1 / test_windows_setup.ps1 / .yml files (no em dashes, curly quotes, smart apostrophes -- they break PS 5.1 CP1252 parsing)
• Conventional Commits format on all commits
• Co-authored-by: Copilot trailer on all commits
• Branch forked from develop