🏗️ Define interactive terminal grid architecture (#717)

[taras]

Why

Quest #717 needs one accepted contract for terminal-grid syntax, concurrent pane
execution, native Agent sessions, provider replacement, foreground attachment,
and complete teardown before its implementation Stories can compose safely.

What changes

Before:

The repository had proof evidence for persistent tmux pane workers, but no
product contract assigning the provider-neutral structure, lifecycle,
durability, host responsibilities, or acceptance evidence.

After:

The architecture and specifications define <Terminal.Grid> and <Terminal>,
the replaceable terminal authority, concurrent pane scopes, atomic startup,
foreground attachment, replay behavior, native session ownership, and the
teardown boundary every provider must satisfy.

How it works

authored grid → provider-neutral layout → concurrent pane leases → host provider
             → foreground view → complete teardown → document continuation

The contract separates core structure and lifecycle from runtime-specific host
assembly. The tmux topology proven by #726 and PR #727 is recorded as evidence,
not installed as product implementation.

Review guide

Start with: architecture.md § Interactive terminal grids

Then review:

1. specs/executable-mdx-spec.md §6.21 and Tier TG
2. specs/native-agent-session-launch-spec.md pane-scoped launch ownership
3. The four Story boundaries and dependency order recorded in Quest: Add interactive terminal grids to executable documents #717

Look carefully at:

• Atomic startup and foreground ownership
• Independent pane settlement
• Durable replay and terminal-session ownership
• The teardown proof required before document continuation

What must stay true

• Public document structure is provider-neutral; tmux remains a host adapter.
• A partial grid is never shown.
• One settled pane does not terminate its siblings.
• The document continues only after every pane and provider resource is gone.
• Closed Test persistent tmux pane workers for <Terminal.Grid> #726 and PR 🔬 Prove persistent tmux pane workers for <Terminal.Grid> (#726) #727 remain evidence rather than product implementation.

How to verify it

• git diff --check proves the contract change has no whitespace defects.
• Stories Describe terminal grids as executable document structure #729–Open terminal grids with tmux in foreground runs #732 own the executable evidence matrix defined as TG1–TG18.

Scope

Included

• Terminal-grid product and architecture contract
• Executable MDX grammar, lifecycle, durability, host profiles, and evidence map
• Native Agent session behavior inside terminal panes

Intentionally unchanged

• No parser, runtime, provider, native launcher, or tmux implementation
• No new proof-of-concept or terminal-holder latency threshold

Risks and limitations

• This is the base of the Quest: Add interactive terminal grids to executable documents #717 PR stack. PR ✨ Describe terminal grids as executable document structure (#729) #733 and the remaining implementation
  Stories are reviewed above it, and the complete feature is verified at the
  final stack head before any layer merges.

Scope confirmation

• Every changed file supports the purpose described above.
• Unrelated cleanup and formatting changes are excluded.
• Generated or mechanical changes are clearly identified.
• The description matches the final diff and verification.

[github-actions]

PR #734: 🏗️ Define interactive terminal grid architecture (#717)

3 files, +777 / -34

Scope

🔴 PR has 811 lines changed. Split into focused PRs.

🟡 811 lines changed. PRs under 400 receive more thorough review.

Structural

✅ No structural bloat detected.

Slop

✅ Slop indicators look low.

Static Analysis

✅ Oxlint found no issues.

Correctness

No extraneous code patterns detected.