docs: README overhaul — landing page + docs/ directory

[Siddhant-K-code]

Summary

README: 1911 lines → 188 lines. No content deleted — relocated to docs/.

Before / after

	Before	After
README lines	1911	188
README sections	93	8
docs/ files	2	9

New docs/ files

File	Moved from README
docs/setup.md	Full setup for all 3 integration paths, complete JSON configs, Cursor/Windsurf configs, security-sensitive repo setup
docs/commands.md	Full flag reference for every command (all 40+ commands)
docs/production.md	Per-backend OTLP setup: Datadog, Honeycomb, Grafana, New Relic, Splunk, Langfuse; behavioral metrics export; OTLP mapping table
docs/server.md	Server-side collector setup, Docker, API reference, multi-agent correlation
docs/integrations.md	Auto-instrumentation for all 6 frameworks with per-framework code examples
docs/vscode.md	Full VS Code extension docs: all 8 commands, settings reference, post-mortem viewer, session browser
docs/security.md	Secret redaction, PII masking, trace anonymization, policy files, shadow AI detection, sensitive repo setup

README structure (new)

1. Header + badges (VS Marketplace badge updated to v0.2.0)
2. Why (trimmed to 2 sentences — the core insight)
3. Install (unchanged)
4. Quick start (3 options, ≤8 lines each, links to docs/setup.md)
5. Command tables (4 groups × ~6 commands, each linked to docs/commands.md)
6. VS Code extension (feature table + 4-step setup, links to docs/vscode.md)
7. Production (3 code blocks + links to docs/)
8. How it works (4 paragraphs, no code blocks)
9. Tests + License + footer links

AGENTS.md changes

• Updated docs/ section table with all 7 new files
• Updated version reference to 0.51.0
• Added README policy (must stay under 300 lines, detail goes in docs/)
• Updated "Adding a new feature" steps to include docs/ updates
• Updated version bump examples to current version

Verification

• All commands in README tables confirmed present in cli.py dispatch table
• All docs/ links in README resolve to existing files
• 1164 tests pass (python -m pytest tests/ -q)
• No content deleted — every section from the old README exists in docs/