feat: ship standalone Forge CLI

[alextacho]

Summary

Forge is now publishable as a standalone Go CLI instead of a Claude plugin toolkit. Users can install or upgrade it with go install, run project-local state operations directly, ask the CLI or MCP server how agents should use it, and publish tagged releases with stamped binaries.

What changed

Area	Outcome
CLI	Adds forge save, forge load, forge reset, forge status, forge instructions, forge version, and forge mcp.
State model	Uses .forge/config.yaml, .forge/templates/, and ignored .forge/snapshots/ for project-local state.
Agent discovery	Exposes read-only MCP tools/resources for instructions, config schema, project status, and snapshot listing.
Publishing	Adds CI, release archives, checksums, semantic tag guidance, and build-time version stamping.
Legacy plugin surface	Removes Claude plugin manifests, slash commands, snippets, and shell scripts that no longer match the product shape.

The implementation keeps mutation behind explicit previews and confirmation unless --yes is provided. Snapshot loading and template resets validate source state before clearing managed paths, preserve symlinks as symlinks, and reject unsafe configured paths.

Test plan

• go test ./...
• go vet ./...
• gofmt check for cmd, internal, and integration
• Local release-style build with ldflags verified forge version output

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