Reframe layout docs around responsive UI and instructional style

[Copilot]

Proposed Changes/Todos

Responsive layout is already a core capability of Terminal.Gui, but the docs emphasized mechanics over the mental model. This updates the layout docs and XML API docs to present Pos/Dim as a declarative, responsive layout system first, with internals and edge cases second, and aligns the updated docs with the required instructional sentence style.

• Reframe the layout guide around responsive composition

  • Add a responsive mental model to docfx/docs/layout.md
  • Lead with relationships (center, fill, anchor, follow another view) instead of low-level taxonomy
  • Rewrite examples to show adaptive form-row style layouts and explain Dim.Fill (to: ...)

• Align API docs with the same mental model

  • Update XML docs in Pos, Dim, and View.Layout to describe:
    • declarative positioning/sizing
    • Frame as the resolved result of layout
    • Layout / SetRelativeLayout as the step that resolves responsive expressions into concrete coordinates

• Clean up stale terminology in layout docs

  • Replace outdated Pos.At (0) references with Pos.Absolute (0)
  • Tighten wording so examples read correctly for both axes and match current API names

• Apply the instructional-sentence rule to the updated docs

  • Rewrite the updated docfx/docs/layout.md prose to use the To [goal], [imperative action]. pattern consistently, including the opening paragraph and mental-model framing
  • Rewrite the related XML API docs in Pos and Dim to use the same instructional pattern
  • Avoid unnecessary When/If you want/need to ... phrasing in the updated reference/how-to text

• Update AI agent instructions so future agents follow the same docs style

  • Add the terse documentation-style rule to AGENTS.md, CLAUDE.md, .github/copilot-instructions.md, and llms.txt

Example of the framing this PR now emphasizes:

Label nameLabel = new () { Text = "Name:" };
Button okButton = new () { Text = "OK", X = Pos.AnchorEnd () };
TextField nameField = new ()
{
    X = Pos.Right (nameLabel) + 1,
    Y = Pos.Top (nameLabel),
    Width = Dim.Fill (to: okButton)
};

This makes the responsive behavior explicit: the label keeps its content width, the button stays anchored to the end, and the text field expands/contracts between them.

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

@copilot apply these rules to the docs you updated.

API Documentation Style Rule – Instructional SentencesUse only this pattern for actions/instructions:

• To [achieve goal], [imperative action].
• To list users, send GET /v1/users.
• To create an item, POST /v1/items with JSON body { "name": "..." }.

Strictly forbid these forms in reference/how-to sections:

• When you want/need to …
• If you want/need to …
• Whenever you …
• To [goal] when you …

Exceptions: Use "when/if" only for real conditions (e.g., rate limits, errors, optional behavior):

• When rate limit is exceeded, return 429.

Rationale (brief): Infinitive-of-purpose + imperative is concise, scannable, professional, and matches top API docs (Stripe, Twilio, OpenAI, etc.). "When you" adds unnecessary verbosity and conditional framing.

update the ai agent instructions to include the tersest version of this so future agents get it right.

[Copilot]

@copilot apply these rules to the docs you updated.

API Documentation Style Rule – Instructional SentencesUse only this pattern for actions/instructions:
...

Applied in c479753. I updated the layout docs and XML docs to use the To [goal], [imperative action]. pattern where I had added instructional text, and added the terse rule to the AI instruction files (AGENTS.md, CLAUDE.md, .github/copilot-instructions.md, and llms.txt).

[tig]

@copilot you did not follow instructions. Literally the first para of layout.md breaks these rules:

Terminal.Gui layout is declarative and responsive. Instead of hard-coding every Frame, you describe how a View should relate to its SuperView, its content, and sibling views. Terminal.Gui then resolves the final Frame whenever layout runs, including after terminal resizes.

[Copilot]

@copilot you did not follow instructions. Literally the first para of layout.md breaks these rules:

Terminal.Gui layout is declarative and responsive. Instead of hard-coding every Frame, you de...

Fixed in b9dc16b. I rewrote the remaining instructional prose in layout.md and the parallel Pos/Dim XML remarks into the required To [goal], [imperative action]. form, including the opening paragraph you pointed out.