Add commit --amend support with UI toggle and refresh product roadmap

[tspython]

Motivation

• Enable amending the last commit from the UI with an explicit, discoverable toggle and keyboard shortcut to make the commit workflow more complete and safer.
• Surface the current product baseline and roadmap in PRODUCT_ROADMAP.md so planning and priorities are aligned with the repository audit.

Description

• Add commit_amend: bool to State and update the commit modal to display amend state and hint text, with Ctrl+A as a toggle in the commit prompt; input handling now passes the control modifier to the commit handler via handle_commit_input(&Key, ctrl: bool).
• Add amend: bool to GitCommand::Commit and include --amend in both the description() string and the actual command arguments when amend is true, plus introduce commit_with_options(...) on GitModel and preserve the old commit(...) API.
• Wire the UI toggle to call git.commit_with_options(&message, false, amend) and update status messages to indicate when commits are amended.
• Replace PRODUCT_ROADMAP.md with an expanded, baseline-grounded roadmap and audit summary to document implemented capabilities, gaps, principles, phases, and an immediate backlog; add one unit test commit_command_description_includes_amend_when_requested to verify the command description includes --amend.

Testing

• Ran cargo test, which executed unit tests including the new commit_command_description_includes_amend_when_requested and all tests completed successfully.
• Verified the commit modal text updates and keyboard Ctrl+A toggling logic by exercising the commit input state in unit/interactive checks (automated unit tests cover the command description change).

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Codex Task