[docs] Reconcile README, roadmap, and issue state with shipped editor behavior

[owainlewis]

Goal: Make the public project docs and open issue state match the behavior that has actually shipped on main.

Context: The local workspace shows editor behavior ahead of the README and some open v0.3 issues, including undo, search, mark/region, kill/yank, directory browsing, and expanded highlighting.
Some of that may still be unmerged local work, so this issue should be done only after the current local changes are either merged or discarded deliberately.
Accurate docs matter because the roadmap is the task queue for future agents.

Proposed approach:

• Start from the latest origin/main after pending work is resolved.
• Compare README, docs/prd.md, docs/roadmap.md, and open GitHub issues against actual merged behavior.
• Update the README current-scope, keybinding, syntax highlighting, and known-limitations sections.
• Update the roadmap only at milestone/task status level.
• Close or adjust open issues only when their acceptance criteria are truly satisfied by merged code.
• Do not claim speculative behavior from the PRD as shipped.

Acceptance criteria:

• README describes the current merged editor behavior accurately.
• Roadmap statuses match open and closed GitHub issues.
• Any shipped v0.3 behavior is not left documented as missing.
• Any still-missing behavior remains clearly listed as a limitation or open issue.
• Open issues [v0.3] Add undo and redo #27 through [v0.4] Add tabs over window layouts #32 are either still valid or explicitly updated/closed with evidence.
• Markdown remains plain and practical.

Verification:

• Run cargo test if code was touched.
• Manually compare README behavior claims against a local smoke run.
• Run gh issue list --state open --limit 100 and confirm the roadmap reflects the open issue set.

Out of scope:

• New editor behavior.
• Rewriting the PRD vision.
• Release notes.
• GitHub Project automation.