docs(packages): unify README structure across cli, lint, parser, mcp

[fyodoriv]

Why needed

The four published packages (cli, lint, parser, mcp) had drifted into different README shapes — different H2 sets, different ordering, different tone — so a reader who clones the repo had to re-orient inside every package directory before they could find Install vs. flags vs. API. The acceptance criteria of docs-package-readme-consistency (the P1 task this PR closes) calls for a single canonical structure across all four READMEs.

What changed

All four package READMEs now share the same H1 + five H2 blocks in the same order:

# <package-name>
## Install
## Use            -- the canonical use case in 5–10 lines
## API            -- CLI flags (cli, lint) or TS types (parser, mcp tools)
## See also       -- spec, root README, sibling packages
## License

Per-package consolidation on top of the structural change:

• cli — collapse the 12 command sub-sections into one API table with flag references; pull the Jira / Linear / GitHub priority-mapping paragraphs into a single line that points at story 06 for the full walkthrough.
• lint — re-derive the validation-rule table from lint.ts so it now also lists the **Blocked** / **Research** / **Last-enriched** rules that already shipped but were missing from the table; merge Usage + CI Integration + Example Output into Use + API.
• parser — surface every exported helper in the API code-fence, list every interface (Task, TaskMetadata, Policy, TaskFile, PickResult) in the same block, and document the pickBestTask ordering contract inline.
• mcp — tighten the tools table so each row is self-describing, fold Setup + Build from source into Use, drop the prose How-it-works bullets that duplicated the table.

Diffstat: +177 / −359 across the four READMEs (net −182 including the TASKS.md hunk that removes the closed task block).

Acceptance check

• All four READMEs share the same five top-level headings (Install, Use, API, See also, License, plus the H1 tagline).
• Cross-links between packages (../cli/, ../lint/, ../parser/, ../mcp/) and to the root (../../README.md, ../../spec.md, ../../docs/user-stories/) all resolve in the working tree.
• Net diff is negative (−154 lines just on the four READMEs; consolidation, not addition).
• npm run build — passes.
• npm test — 407/407 tests pass across cli (183), lint (43), mcp (103), parser (78).
• npm run lint — Checked 1 file(s), found 0 error(s).
• npx -y @tasks-md/lint TASKS.md — Checked 1 file(s), found 0 error(s).

closes docs-package-readme-consistency

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🤖 Written by an agent, not Fyodor. Ping me if this looks off.