docs(readme): fix the broken logo and state where Stella sits next to Oxagen

[macanderson]

docs(readme): fix the broken logo and state where Stella sits next to Oxagen

The README header pointed at docs/brand/logo/svg/lockup-color-light.svg,
which #6473 (the house-system adoption) deleted along with the rest of the
comet kit, so GitHub rendered a broken image above the badges. It now uses
the vendored stella-wordmark-{dark,light}.svg pair behind prefers-color-scheme
sources, with the light file as the fallback.

The copy is brought onto the same position the site and the Oxagen repo
already state:

• The tagline is the site's h1 ("a terminal coding agent that proves its
  work finished") in place of "Reference Grade Agent Loop".
• A short "Stella and Oxagen" section states the split from ADR-043 and
  ADR-053 on the Oxagen side: Stella runs the agent, Oxagen governs the run;
  Stella is the coding agent and the CGP/trace reference implementation,
  Oxagen is the governance plane with no agent loop; the two meet at the
  opt-in evidence drain and at stella-serve, which Oxagen's in-app assistant
  runs on. It also names the shared house brand and the stella* mark.
• The workspace-layout paragraph that called crates/stella-tui/src/palette.rs
  the hand-maintained palette mirrored by tokens.css is replaced with what is
  true since v5.0 and brand: adopt the Oxagen house system — palette, marks, and the face #6473: design/tokens/stella-tokens.json is normative,
  gen-tokens.py emits the terminal tokens and the CSS, and
  sync-brand-assets.mjs vendors the marks from the house kit.

Verified: check-doc-links, check-invariants, check-command-docs,
check-brand-case, check-line-citations, check-rendering-facts and
make tokens all pass; every relative link and image path in the README
resolves.

Summary by Sourcery

Update the README to restore the logo, accurately position Stella alongside Oxagen, and document the current branding workflow.

Bug Fixes:

• Fix the README logo rendering by using the vendored dark and light Stella wordmarks with a light-theme fallback.

Enhancements:

• Clarify Stella’s relationship with Oxagen, including their distinct responsibilities, integration points, and shared branding.
• Update the workspace documentation to describe the normative token source and generated brand assets accurately.

Documentation:

• Refresh the README tagline, privacy wording, product positioning, and workspace layout guidance.

[github-actions]

SCR-003 DoD check waived by the closes-nothing label — this PR closes no issue by design.

[sourcery-ai]

Reviewer's Guide

Updates README branding and product positioning: fixes the broken logo with responsive vendored wordmarks, explains the Stella/Oxagen split and integration points, and documents the current generated token and brand-asset workflow.

File-Level Changes

Change	Details	Files
Replace the broken README logo reference with responsive, vendored Stella wordmarks.	Use dark and light SVG sources selected by prefers-color-scheme. Provide the light wordmark as the fallback image and update its accessible label.	README.md
Align README positioning and product messaging with the Stella/Oxagen architecture.	Update the tagline and privacy wording to match the site. Add the Stella and Oxagen responsibilities, integration seams, branding relationship, and referenced documentation links.	README.md
Correct the README description of the token and brand-asset workflow.	Document the normative JSON token source and generated terminal/CSS outputs. Document house-kit asset synchronization and the associated validation commands.	README.md

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