docs: expand motivation section in README

[robbycochran]

Summary

• Rewrites the "Why this exists" section to better articulate the problem and design constraints
• Adds the CI-tested harness insight: if the developer harness isn't live-tested in CI, the developer experience can't be maintained as upstream dependencies change
• Adds the local-to-automated path: same YAML config moves from laptop to OCP by changing one field

Changes

Expanded from 3 paragraphs to 5 in the "Why this exists" section. No other files changed.

Summary by CodeRabbit

• Documentation
  • Clarified project purpose and architecture, explaining how the harness bridges interactive local development with automated CI validation workflows across multiple deployment targets using unified configuration.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: bb87d3a0-d0ef-4e59-95e1-5017a20f0cef

📥 Commits

Reviewing files that changed from the base of the PR and between 541b5b7 and 374d5a9.

📒 Files selected for processing (1)

• README.md

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Walkthrough

The "Why this exists" section in README.md is expanded from 2 lines to 6 lines, adding explanation of OpenShell's runtime role versus the harness workflow layer, a CI validation design constraint, lifecycle coverage across targets, and the --attach to --task workflow transition.

Changes

README Documentation Update

Layer / File(s)	Summary
Expanded "Why this exists" section README.md	Replaces a brief two-line description with six lines that distinguish OpenShell's secure sandbox runtime from the harness workflow layer, state CI validation as a core design constraint, describe harness-openshell lifecycle coverage across multiple targets, and document the transition from --attach local runs to --task headless runs using the same harness YAML artifact.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch rc-readme-why-section

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