docs(readme): improve README with better structure, CLI reference, and cost model

[mayakost]

Summary

• Rewrites the README with a clear feature table, scannable sections, and improved navigation for new users
• Adds a complete step-by-step quick-start section with runnable code snippets (clone → install → deploy → first task)
• Adds a bgagent CLI reference section covering all commands and common options
• Adds a cost model table with scale estimates
• Adds a documentation index table linking to all guides and design docs
• Adds additional badges (license, TypeScript, Python) to the header
• Preserves all existing content while reorganizing it for clarity

Build and test results

Commands run:

• mise run //docs:sync — ✅ passed (no stale mirrors)
• mise run build — ❌ pre-existing failure unrelated to this PR: //agent:typecheck fails due to botocore/boto3 unresolved imports in the Python type checker (ty); this failure was present before any changes (confirmed in the task setup notes: "Initial build FAILED before agent changes"). The README change does not affect agent code, CDK, or CLI.

Decisions made

• Kept the existing docs/imgs/ABCA.png logo and architecture image (docs/imgs/abca-arch.png).
• Surfaced the Claude Code plugin as the recommended path since it's the fastest onboarding experience.
• Included just enough of the Quick Start to let a user submit their first task without leaving the README, while still linking to the full guide for depth.
• The CLI reference is a concise summary, not a full manual — the User Guide has the full details.

Agent notes

What went well:

• The project has excellent documentation (AGENTS.md, DEVELOPER_GUIDE.md, ARCHITECTURE.md, QUICK_START.md, ROADMAP.md) that made it straightforward to produce a well-informed README without guessing at capabilities.
• The docs sync script is fast and clearly scoped (only docs/guides/ and docs/design/ — not README.md), so no regeneration was needed.

What was difficult:

• The mise run build has pre-existing failures in //agent:typecheck due to a Python type checker (ty) not finding boto3/botocore even though they are installed in the venv. This is a known environment issue that predates this PR.
• The //docs:build also fails pre-existing (missing astro binary), but the sync step (//docs:sync) that matters for CI succeeds.

Patterns discovered:

• MISE_EXPERIMENTAL=1 is required for all namespaced mise //pkg:task commands.
• Generated docs live in docs/src/content/docs/ and must not be edited directly — docs/scripts/sync-starlight.mjs generates them from docs/guides/ and docs/design/.
• Commit format is type(module): description (Conventional Commits) per CONTRIBUTING.md.

Suggestions for future tasks:

• The pre-existing //agent:typecheck failure should be investigated — it appears to be a ty configuration issue where the venv path is not being found correctly despite the packages being installed.
• The //docs:build failure (missing astro binary) suggests yarn install may not have run properly in the docs/ workspace in this environment.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.