docs: HARNESS, INVARIANTS, BOUNDARIES, DEVELOPMENT, EVAL_HARNESS, SECURITY, ARCHITECTURE skeletons

[constk]

Problem

The harness is invisible without docs that explain it. Users cloning the template need to understand the rules they're inheriting.

Proposed solution

Adapt seven docs from Teller: HARNESS.md (umbrella + three principles, frame the harness AS the product), INVARIANTS.md (5 portable rules: StrictModel forbid, layer flow, models depends on nothing, SQL parameterisation, input bounds; numbered slots 6+ for users to add), BOUNDARIES.md (layer diagram updated for the scaffold's directories), DEVELOPMENT.md (Python 3.14, Node 24, uv, Docker, branching, justfile, version-bump rule), EVAL_HARNESS.md (pattern + opt-in), SECURITY.md (generic threat model + defence-in-depth mapping), ARCHITECTURE.md (scaffold-level system diagram).

Acceptance criteria

• Every doc references concrete files in this template, not Teller paths.
• INVARIANTS.md numbers 1-5 are filled; 6+ are explicitly marked "[your project adds here]".
• DEVELOPMENT.md documents the version-bump rule (Python / Node LTS / React).
• No mentions of "Teller", "financial", or any Teller-specific identifier (grep verified).

Priority rationale

High: docs are the artefact a recruiter or engineer reads first. They have to be tight.

Depends on

#2, #9, #15

[constk]

Merged in PR #61.