docs: overhaul Handler documentation

[alDuncanson]

Summary

Overhauls the Handler documentation for the v0.1.22 release so the docs better match the current CLI, enterprise A2A workflows, and MCP/doc-server capabilities.

Key changes:

• Refreshes the README, quickstart, landing page, and existing guides around the current Handler workflow.
• Adds a complete CLI reference covering global options, command metadata, card/message/task/session/server commands, and integration commands.
• Adds enterprise A2A guidance for OAuth2 client credentials, gateway-fronted agents, structured output, and timeout tuning.
• Adds Handler docs MCP guidance for using the documentation server from MCP-compatible clients.
• Expands message/task/session docs with streaming, JSON/NDJSON output, raw JSON input, continuation, task operations, and push notification examples.
• Updates docs navigation and TUI assets.
• Fixes session list structured output to emit a single JSON array of sessions instead of one JSON object per session.

CLI smoke testing

Exercised the documented v0.1.22 CLI surface against an OAuth2-protected A2A server and verified the command behavior matches the updated docs.

Commands exercised included:

• handler --version, handler version, and command help output
• server list, server show, server validate, temp-dir server add/remove
• card get, card validate --server, card validate --file
• schema, describe message send, describe task get, describe server add/remove
• message send --text, message send --continue, message send --json, custom -H header
• message stream in text and NDJSON modes
• task get, task get --history-length, task resubscribe, task notification get/set, task cancel
• session list, session show, session clear

Expected server capability/state limitations were observed for unsupported operations such as resubscribe, push notifications, and canceling an already-completed task.

Verification

• uv run ruff check .
• uv run ruff format --check .
• uv run ty check — exit 0; reports two existing warnings in tests/server/test_card.py
• uv run pytest — 462 passed
• GitHub CI/CodeQL checks are passing