Document the portable playground product vision

[taras]

Why

The product vision predates the playground direction and does not explain how
agent authoring should relate to local and browser execution. This leaves room
for a server-only playground, host-specific workflow formats, or silent remote
fallbacks that would undermine workflow portability.

What changes

Before:

The vision describes readable, reliable, observable, and composable Markdown
agents without defining the playground or a portability promise.

After:

The vision establishes the playground as the primary authoring surface, with
the same Markdown project and workflow semantics available locally and entirely
in the browser. It defines explicit capability boundaries, constrained trials,
portable evidence, immutable revisions, and separately authorized real runs.

Review guide

Start with: specs/markdown-agents-vision.md

Then review:

1. The expanded north star and new Portable product quality.
2. The human-agent playground authoring loop.
3. The boundary between portable semantics and host-specific capabilities.
4. The added portability questions in the design test.

Look carefully at:

• Browser and local hosts are peers; a remote host is explicit and never a
  silent fallback.
• Markdown remains canonical; semantic views do not create hidden workflow
  representations.
• Exported evidence may start a linked history but never revives the original
  execution.

What must stay true

• The executable MDX specification remains authoritative for implemented
  behavior; this document remains non-normative product direction.
• Portability preserves workflow meaning without claiming that every host has
  identical authority or integrations.
• Host-specific behavior enters through contextual implementations rather than
  runtime detection in shared production modules.

How to verify it

• git diff --check proves the documentation change introduces no whitespace
  errors.
• Read the design test and confirm it now rejects undocumented host-specific
  project formats and capabilities.

Scope

Included

• Product direction for portable local and browser playgrounds.
• The authoring, trial, revision, authorization, and evidence lifecycle.
• Design criteria for future playground work.

Intentionally unchanged

• Runtime behavior and the executable MDX specification.
• The illustrative agent syntax already recorded in the vision.
• Implementation status, issue tracking, and delivery sequencing.

Risks and limitations

• This PR settles product direction; it does not claim the complete playground
  is already implemented.

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 test results.

[github-actions]

PR #650: Document the portable playground product vision

1 files, +74 / -3

Scope

✅ PR scope looks good.

Structural

✅ No structural bloat detected.

Slop

✅ Slop indicators look low.

Static Analysis

✅ Oxlint found no issues.

Correctness

No extraneous code patterns detected.