docs(editor): clarify agent workflows and upcoming release features

[fcoury-oai]

Why

Red's public documentation and onboarding understate its editor-aware Agent, focused inline assistance, and Vim compatibility. Some existing copy also implies that all Agent writes require manual approval, which incorrectly describes the distinction between saved full-Agent edits and unsaved inline edits.

What Changed

• Reposition README.md around editor-aware Codex conversations, inline assistance, Vim fluency, and the bundled terminal-editor workflow while clearly separating published v0.6.0 behavior from features currently available only on main.
• Expand docs/GETTING_STARTED.md, docs/AGENT_WORKFLOW.md, and docs/VIM_COMPATIBILITY.md with multi-cursor commands, source-linked Agent explanations, conversation-specific model selection, external-file conflict recovery, completion behavior, formatting, and exact inline targeting.
• Correct onboarding and welcome-screen copy: full Agent edits are revision-checked and saved through Red, while inline edits remain unsaved and undoable; the guided Agent demonstration stays isolated in a disposable practice buffer.
• Archive the complete substantive release-communications and launch-strategy discussion in docs/RELEASE_COMMUNICATIONS_CONVERSATION.md, excluding runtime/tool metadata, credentials, and machine-specific absolute paths.

Companion release automation: #350.

How to Test

1. Run python3 scripts/readme_release.py --check; the README should continue to match the published package version without advertising upcoming main features as released.
2. Run cargo test -p red --lib tutorial::tests and cargo test -p red --lib ui::welcome::tests; all five tutorial tests and four welcome tests should pass, including the full-Agent versus inline save-boundary assertions.
3. Read the new README and getting-started sections; verify multi-cursor editing, source annotations, model selection, and external-file recovery are explicitly labeled as upcoming, while Space i describes enclosing-function targeting and exact Visual selections.
4. Inspect the archived strategy document; confirm it contains the substantive user/assistant discussion but no internal citation metadata or absolute /Users/... paths.