docs: STE100 pass over README, CONTRIBUTING, and docs/

[willkg]

Part of #165.

Replaces #183, which GitHub closed when its head branch was renamed from readme-fix to this one.

An ASD-STE100 (Simplified Technical English) pass over seven documents, plus the corrections that reviewing them turned up.

README.md

• STE100 rewrite: short sentences, active voice, no gerund headings, no em dashes. Every code block, link target, and table row is kept.
• Restores details the rewrite dropped or changed: round-trip reassurance, the Homebrew completions note, the --root scope (diff redirects .env too, check honours the per-file root), find returning folder ids, and the .env warning firing on any group or world bit.
• Fixes claims that were already wrong on main: moving an asset keeps its attachment (the name is the base name), the error-code list was missing CONFLICT, the schema example was missing space-info and user-info, and the create workflow used two different filenames.
• Says that the divergence refusal and the unchanged-body skip both need a markfluence.yaml, and tells one-file-at-a-time users what they give up without one.

CONTRIBUTING.md

• STE100 rewrite. It keeps the welcome and the reason for recording Confluence findings in their original shape on purpose.
• Lists docs-check among what make check runs.

docs/guarantees.md

• STE100 rewrite of the prose. Every id, label, status, code identifier, and reference is unchanged.
• Corrects three stale claims against the code: the L3 statement (identity follows the file name, not the location), the orphaned-attachments reason, and the withinRoot paragraph in Symlinks. It also records the upload race (Attachment upload reads outside the os.Root that checked it #186) as a known limit of S2.

docs/json-output.md

• STE100 rewrite. Checked against the schema, the review fixes: CONFLICT added to the error codes; export's status verbs (it never emits failed) and its multi-page results under --depth/--space; when body_changed and page_status are null; space-info, user-info and children --space reporting failure on stderr; an empty command on a bad-flag error.

docs/markdown_file.md

• STE100 rewrite. The long notes for parent, labels, page_width and page_status move out of the field table into a subsection each.
• Review fixes: create refuses a --space that disagrees with frontmatter, so flag > frontmatter holds only for width; title is optional for update; page_status is a single-value field; create writes page_width back, so a later update asserts it; a .md parent can be a page published earlier; doc links resolve to any .md under the root; <!-- bg:COLOR --> is a second comment directive.

docs/releasing.md

• STE100 rewrite; the workflow's run in step 4 becomes a numbered sequence.
• Review fixes: the binary prints the version without the v; the Homebrew check runs the cask binary by full path so a make install build on PATH cannot answer; the rehearsal's go mod tidy and make completions hooks; make check and the final publish are workflow steps, not goreleaser; recovery when no draft exists, or when only the publish step failed.

docs/root-model.md

• STE100 rewrite. The four headings other documents link to are unchanged.
• Review fixes: update does depend on the log for its two checks; --no-persist still writes a log line; a link outside the root is LINK BROKEN, and check exists; an image resolves relative to its page; an attachment's name is its base name, so moving an image keeps it; diff also redirects .env under --root; preflight converts before reserve; info is page-info.

Each of these four files has two commits: the rewrite, then the fixes from a code review of it.

_plans/048_principles.md (on main) plans reshaping guarantees.md into design principles with no statuses, so these status corrections are the last of that kind.