docs(economics): opt-in usage-data sharing + K/M number formatting

[potiuk]

What & why

Adds a voluntary, opt-in program for adopters to contribute anonymised
token-economics data, and makes the economics page easier to read.

Usage-data sharing (opt-in)

• docs/mode-economics.md — new "Sharing your usage data (opt-in)"
  section: how to opt in, the session-end question, submission as a PR to
  apache/magpie-economics,
  what the payload contains, and the privacy exclusions. Wired into the
  ASF-inference-endpoint section and cross-references.
• AGENTS.md — new cross-cutting rule "Voluntary usage-data sharing
  (opt-in)": when — and only when — the adopter has opted in, a skill
  session ends by asking whether to submit; never auto-submits; hard rule
  that the payload carries no PII and no subscription/billing data.
• projects/_template/project.md — new economics_data_sharing key
  (default off).

Privacy contract. The data payload is anonymous — per-skill token
counts, model/model-class, public repo name, and a theoretical cost figure;
no personal information, no subscription/plan/seat/quota/billing data. The
one honest caveat, stated in all three files: the submission is a PR opened
from the maintainer's own GitHub account, so their account is attached
as author (inherent to any PR) — their own identity, disclosed voluntarily,
never third-party data.

Readability

• Reformatted every token count on mode-economics.md to K/M notation
  (30 000 → 30K, 1 500 000 → 1.5M), with a one-line key.

The submission PR template lives in the now-bootstrapped
apache/magpie-economics repo.

Validation

• doctoc TOCs regenerated; markdownlint, typos, lychee, check-placeholders all pass (pre-commit)
• spec-validate OK; skill-and-tool-validate clean (one pre-existing unrelated warning)
• lychee --offline --include-fragments: 0 errors across all new cross-file anchors

Generated-by: Claude Code (Opus 4.8)