feat: companion plugin with 4 slash commands: https://github.com/riboseinc/claude-memory-files/issues/1 - #12

Implements ticket #1 (#1);
PR 8 of the delivery sequence.

Lands the companion Claude Code plugin that wraps the curl installer in
slash-command UX:

  - .claude-plugin/marketplace.json (at repo root): declares this repo as
    a Claude Code plugin marketplace. Single plugin: memory-files-tools.

  - tools/plugin/.claude-plugin/plugin.json: plugin manifest.

  - tools/plugin/commands/install-memory-file.md
    /install-memory-file <slug> [--project <dir>] [--ref <sha>]
    Looks up the slug in the catalogue index, shows frontmatter, runs
    curl | bash on tools/install.sh from main, reports the install result.

  - tools/plugin/commands/remove-memory-file.md
    /remove-memory-file <slug>
    Reads the manifest, shows what will be removed, asks for confirmation,
    runs tools/uninstall.sh. Surfaces backup paths for settings-fragment
    un-merge.

  - tools/plugin/commands/list-memory-files.md
    /list-memory-files [--tag <tag>] [--scope <s>] [--team <t>] [--installed]
    Fetches _meta/index.json, filters, renders as a per-category list,
    suggests next-action slash commands.

  - tools/plugin/commands/submit-memory-file.md
    /submit-memory-file <path-to-local-file.md> [--category <c>]
    Walks the user through frontmatter scaffolding (with controlled-tag
    vocabulary lookup), clones the repo as a sibling per the
    sibling-clones convention, creates a feature branch, runs the
    validator, commits via -F file per github-content-via-file, and opens
    the PR with --assignee opoudjis and the issue-URL-in-title convention.

The plugin ships only the slash commands and the manifests; install.sh and
uninstall.sh are fetched from main on each invocation so the installer
logic stays current.

Install:
  /plugin marketplace add riboseinc/claude-memory-files
  /plugin install memory-files-tools@claude-memory-files

Smoke test #5 from the plan: validated locally that the plugin tree is
well-formed (marketplace.json + plugin.json parse cleanly via jq; slash
command frontmatter follows the Claude Code spec); the full end-to-end
test requires installing via `/plugin install` which the maintainer can
do interactively post-merge.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Adds two checks to validate.yml that gate PR 8 and any future plugin work:

  - marketplace.json + plugin.json must be valid JSON; every declared
    plugin source path must exist and contain a valid plugin.json.
  - Every slash command markdown in a declared plugin must have YAML
    frontmatter with a non-empty 'description:' field.

Closes a CI gap that PR 8 would otherwise leave: plugin manifests and
slash command frontmatter had no automated validation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>