Add CONTRIBUTING guide (authoring skills/agents/stacks) and version/release policy

[prorochestvo]

Context

The kit's entire value proposition is "improve centrally, propagate via
/plugin marketplace update." Yet there is no documented contract for extending it,
and the versions have already drifted: marketplace.json metadata.version is 1.0.0
while pipeline is 1.4.0, knowledge 1.1.0, stack-go 1.1.0, stack-flutter
1.0.0 — with no stated policy for what a bump means or how a consuming project knows to
update. Since the plan is to keep expanding this, the authoring path needs to be written
down before contributors (including future-me on another machine) diverge.

Proposal

Add CONTRIBUTING.md covering:

• Adding a skill — directory + SKILL.md frontmatter contract: when to use
  paths:-scoped loading vs description-matched loading, and how to write a description
  that actually triggers.
• Adding an agent — frontmatter (name, description with examples, model,
  color, memory), and the role-boundary discipline the four current agents follow.
• Adding a stack — the full checklist: new plugin, templates/<stack>/ dir,
  sync-permissions/stack-<x>.json, install.sh case, marketplace.json entry.
• Make install.sh's stack set data-driven (derive from templates/*/ or a small
  manifest) so adding a stack is not "edit the hardcoded go|flutter case in a shell
  script."
• Version & release policy — when to bump a plugin version vs the marketplace
  version, whether marketplace.json metadata.version tracks anything, a
  CHANGELOG.md, and optional git tags per release.

Acceptance criteria

• CONTRIBUTING.md merged with the three "how to add X" sections.
• Version policy documented; the marketplace-vs-plugin version relationship stated
  (and the current drift reconciled or explicitly declared independent).
• install.sh no longer hardcodes the stack list — or the guide names the single
  place to edit when adding one.
• CHANGELOG.md seeded.