docs: rewrite the dropbox docs in ASD-STE100

[jshvn]

Consistency review, phase 5, for dropbox: C20, C33, C41, C43, C47, C49, C75 and C77 (all Markdown in ASD-STE100).

• CLAUDE.md has the six-mirror shape (C33), with the clock row and the offline verb after the phase 4 refactor (C20).
• README.md: the pipeline diagram (C41), the healthcheck before a first fill (C43), operation (C47). The session rules link to lib (C49).

Merge after the lib docs PR (lib#23): links here go to sections that it adds.

Test plan

• Fact check (spec §4) by a second agent: 255 facts, 0 missing; its one finding fixed
• task check after the rebase onto main: render matches render.txt
• task offline: 165 tests, ruff clean
• Both files render through GitHub's Markdown API