docs: lead README with governance; remove build-hero + images

[stackbilt-admin]

Summary

The repo's own description already reads "Local-first AI agent governance CLI. Modular .ai/ context system (ADF) replaces monolithic CLAUDE.md files." But the README opened with a stackbilt run scaffolding demo and stackbilt-charter-4.png hero, contradicting that framing. This PR aligns the README with the repo's identity — governance-first, with scaffolding flagged as deprecated and moving to @stackbilt/build.

Changes

README opening

• Replace "Your first Cloudflare Workers project shouldn't take a week" + charter run Quick Start with a governance-first opener built around charter bootstrap --yes.
• Rewrite "What you get" to lead with ADF, metric ceilings, blast/surface analysis, drift/audit, and the MCP server. The ecosystem-packages table is demoted to a one-line inline reference (still discoverable, no longer the hero).

Auth line

• Drop **Free to try.** \charter login --key sb_live_xxx` ...` — scaffold auth is moving out via RFC RFC: Extract commercial surface from @stackbilt/cli into @stackbilt/build #112 and has already been env-var-authed as of feat(cli): STACKBILT_API_KEY env var auth; deprecate charter login #111.

Commands reorder + deprecation

• New order: Govern → ADF → Analyze → Build (last).
• Build section now has a callout flagging the four commands as deprecated and pointing at RFC #112 for the @stackbilt/build extraction.
• charter login --key line annotated with "prefer STACKBILT_API_KEY env var."

Image cleanup

• Remove stackbilt-charter-{2,3,4}.png (scaffolding hero series — named for the binary that's moving out).
• Remove ADF_1.png and its reference in packages/adf/README.md.
• Can be re-added with governance-aligned visuals in a follow-up when those exist.

What this does not change

• No functional code changes.
• charter login, charter run, charter architect, charter scaffold still work — they're documented as deprecated, not removed. Removal happens in Charter 1.0 per RFC RFC: Extract commercial surface from @stackbilt/cli into @stackbilt/build #112 Phase 4.
• Modular packages table, Development, License sections preserved verbatim.

Test plan

• Render the new README locally and skim for typos or broken links.
• Verify no remaining references to removed images: grep -r 'stackbilt-charter\|ADF_1' --include='*.md' returns zero hits.
• Spot-check the deprecation callout on the Build section reads correctly on GitHub.

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