docs: tighten README — cut duplication, collapse prior art + repo layout

Full structural pass on the README addressing five issues that showed up
on a fresh read:

1. Privacy framing was repeated in 5 places (header, "What You Don't Get",
   Roadmap hard line, separate Security section, scraper note). Collapsed
   to one honest block in the subtitle + the existing SECURITY.md link.
   Separate Security section removed entirely — it duplicated the header.

2. Prior-art comparison (~60 lines of project links) sat between the pitch
   and the Quickstart. Moved it below the fold inside a <details> block so
   readers who want to install don't have to scroll past a bibliography.

3. Repo layout ASCII tree (~24 lines) moved into a <details> block for
   the same reason. Useful for contributors, skipped by first-time readers.

4. "Two ways in" (Lite vs Full) and "Bringing in transaction data" (CSV
   vs Truthifi) presented orthogonal axes as if they were the same choice.
   Consolidated: one Lite quickstart at the top, one table comparing the
   two data-source paths, one Full-setup block lower down.

5. finance-perspective pitch was a 70-word bullet inside a list. Pulled
   into its own named section ("The one skill this repo exists for") so
   the headline value prop gets landing-page treatment.

Also removed:
- "What You Don't Get" section (negation of what the header already says)
- "HENRYs" jargon from the audience description
- The `sync-local` rename parenthetical in the skills list (belongs in
  CHANGELOG, not in the evergreen README)
- Three-line Philosophy section collapsed into one paragraph

Net effect: raw file is 97 lines shorter pre-expansion; rendered first
scroll drops from ~172 lines to ~95. Every idea is preserved.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: bn-wright