Add first-launch onboarding hint

[forketyfork]

Issue

New users need a concise introduction to Architect's core terminal shortcuts and AI-agent hook setup. PRs #177 and #178 implemented overlapping versions of this behavior, but both were based on an old main and are now superseded.

Solution

Render a faint four-line onboarding hint in the first focused live terminal. It is drawn by the scene renderer rather than sent through the PTY, so it does not affect shell history, terminal state, or agent sessions.

Persist onboarding_shown after the hint is actually drawable, preserve the value across all current persistence formats, and treat older persistence files without the field as already seen. A genuinely new installation still starts with the hint enabled. Regression tests cover persistence round-tripping, migration behavior, and renderer gating. Documentation describes the user-visible behavior and persisted field.

Context

This PR consolidates the best parts of #177 and #178 on top of the current origin/main.

Test plan

• On a fresh configuration, launch Architect and confirm the faint hint appears in the first terminal with the Cmd+N, Cmd+W, Cmd+Enter, Escape, and hook instructions.
• Relaunch Architect and confirm the hint is not shown again.
• Confirm the hint is visual-only and does not appear in shell history or terminal scrollback.

[Copilot]

Pull request overview

Adds a first-launch onboarding hint rendered directly by the scene renderer (not via PTY output) and persists a new onboarding_shown flag so the hint is only shown for genuinely new installs while older persistence files are treated as already-seen.

Changes:

• Render a faint 4-line onboarding hint in the focused, live first terminal session and mark it “displayed” only once it was actually drawable.
• Persist onboarding_shown across all supported persistence TOML formats, treating missing values as already shown for upgrades.
• Update docs and add regression tests for the renderer gating and persistence migration/round-tripping.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/render/renderer.zig	Renders the onboarding hint in the first focused live terminal and reports when it was drawable.
src/config.zig	Adds onboarding_shown, migrates missing values to “already shown”, and tests persistence behavior.
src/app/runtime.zig	Threads onboarding state into rendering and persists onboarding_shown after a drawable render.
README.md	Documents the user-visible “first launch hint” behavior.
docs/configuration.md	Documents the new onboarding_shown field and its migration semantics.
docs/ARCHITECTURE.md	Updates persistence/renderer documentation to mention onboarding state/hint.

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