Add cross-client installer bundling skills + DocumentDB MCP server

[xgerman]

What

Replaces the broken npx skills add Azure/documentdb-agent-kit placeholder
with real one-line installers that, in a single command, wire both this
kit's skills and the microsoft/documentdb-mcp
server into every detected MCP client.

# macOS / Linux
curl -fsSL https://raw.githubusercontent.com/Azure/documentdb-agent-kit/main/install.sh | bash -s -- --uri "<conn>"

# Windows
$env:DOCUMENTDB_URI = "<conn>"
irm https://raw.githubusercontent.com/Azure/documentdb-agent-kit/main/install.ps1 | iex

Why

The kit was just announced with the line
npx skills add Azure/documentdb-agent-kit, but that command does not exist:
there's no skills package on npm, and the repo had no installer of its own.
Customers clicking the link in the announcement would have hit an error.

This PR makes the announcement true and goes further — the same install
command now also wires up the upstream DocumentDB MCP server, so customers get
both tooling and guidance in one step.

What's in the box

File	Change
install.sh	New — Bash installer (macOS / Linux). 388 lines, no runtime deps beyond git + Node 20+.
install.ps1	New — PowerShell installer (Windows + cross-platform pwsh 7+).
skills/mcp-setup/SKILL.md	Rewritten — the previous version walked users through setting $DOCUMENTDB_URI in a shell profile, which is not how the current upstream MCP server is actually configured. New version is built around the actual upstream contract: per-client MCP config file with CONNECTION_PROFILES JSON, TRANSPORT=stdio, ALLOW_UNAUTHENTICATED_STDIO=true. Includes a per-client config-file table for Claude Code / Desktop / Cursor / Copilot CLI / Gemini CLI / VS Code.
README.md	Install section rewritten: real one-liner, install matrix (what gets written where, per client), requirements, all flags, verify steps, uninstall command, manual fallback.
AGENTS.md	1-line edit — mcp-setup row description updated to match.
CHANGELOG.md	New top entry.

Client coverage

Client	MCP entry written to	Skills installed at
Claude Code	~/.claude.json	~/.claude/skills/ (symlinks)
Claude Desktop	claude_desktop_config.json (per-OS)	Claude/skills/ (symlinks, if dir exists)
Cursor	~/.cursor/mcp.json	— (Cursor Rules are project-local)
GitHub Copilot CLI	~/.copilot/mcp-config.json	— (copy AGENTS.md per-project)
Gemini CLI	~/.gemini/settings.json	— (use GEMINI.md per-project)

Safety properties

• Existing entries preserved. Both installers add (or update) a single
  DocumentDB entry; other servers and unrelated top-level keys are kept.
• .bak timestamped backups before every JSON edit. Atomic writes
  (tmp+rename), so a crash mid-write can't corrupt a config file.
• Idempotent. Re-running the installer does not duplicate entries.
• Selective uninstall. --uninstall removes only the kit's symlinks +
  DocumentDB MCP entries. Foreign symlinks and other servers are untouched.
• Single-element array preservation. Both python3's json module and the
  PowerShell pipeline (with the explicit ,@(...) wrapping helper) keep
  one-element args arrays as arrays — so installing alongside an MCP server
  whose args is ["one-thing"] doesn't corrupt it into a string.

Flags

--uri <conn>        DocumentDB / MongoDB connection string
--yes               Non-interactive
--dry-run           Print planned changes; write nothing
--uninstall         Remove MCP entries + skill symlinks + ~/.documentdb-agent-kit
--clients <list>    Comma-separated subset of supported clients
--skills-only       Skip MCP server install
--mcp-only          Skip skill linking
--mcp-ref <ref>     Git ref of microsoft/documentdb-mcp (default: main)
--kit-ref <ref>     Git ref of Azure/documentdb-agent-kit (default: main)
--profile <name>    CONNECTION_PROFILES key name (default: default)

(PowerShell uses -Uri, -Yes, etc.)

Testing performed

End-to-end against fake client configs in sandboxed $HOME on macOS, for
both Bash and PowerShell installers, verifying:

• ✅ Detects all 5 clients when their config dirs exist
• ✅ Existing MCP servers preserved across install
• ✅ Top-level non-mcpServers keys preserved
• ✅ Multi-element args arrays preserved
• ✅ Single-element args arrays preserved (regression-tested — was a real
  PowerShell bug, fixed before this PR)
• ✅ Re-running install does not create a duplicate DocumentDB entry
• ✅ Skill symlinks point at the kit's skills/<name>/ directories
• ✅ --uninstall removes only kit entries; foreign symlinks left in place
• ✅ --uninstall removes ~/.documentdb-agent-kit/
• ✅ Pre-existing ExistingServer and keep: me top-level keys survive both
  install and uninstall
• ✅ All 16 skills still pass scripts/validate-skills.ps1

Known limitations / follow-ups (not in this PR)

• Upstream is currently private. microsoft/documentdb-mcp is a private
  repo at the time of writing. The installer assumes upstream goes public
  before customers run it. Once upstream is public, no installer change is
  needed.
• MCP ref is main by default. Once upstream cuts release tags, we should
  bump the default --mcp-ref to a pinned tag for stability.
• DXT bundle / Claude Code plugin / skill-serving wrapper MCP server are
  better long-term shapes for some clients; they remain out of scope for this
  PR. The installer is the cross-client lowest-common-denominator that works
  today.

How to verify

# Show what the installer plans to do (writes nothing)
./install.sh --dry-run --uri "mongodb://localhost:27017" --yes

# Full install with local MongoDB / DocumentDB
./install.sh --uri "mongodb://localhost:27017" --yes

# Uninstall
./install.sh --uninstall --yes

After install, fully quit and reopen each configured client, then ask the
agent: "list databases using the DocumentDB MCP server with connection_profile 'default'".