feat(lint): add opt-in --require-prereg rule for rule-#9 fields

[fyodoriv]

Why this is needed

spec.md § rule-9-pre-registration-block (PR #78) documents the five rule-#9 fields — Hypothesis, Success / Acceptance, Pivot, Measurement, Anchor — but there's no deterministic gate that enforces them. Without the gate, the rule is just spec — operators silently drop Success or Pivot and the rule degrades to wish-list. This PR ships the gate.

The pattern prevents post-hoc fishing for flattering metrics (Munafò et al. 2017) and pre-registers the give-up criterion (Ries 2011, pivot-or-persevere) by forcing the team to declare what observable a non-trivial change is expected to move before the code is written.

The check has been running successfully in Minsky's repo as a repo-local script (`scripts/check-rule-9-tasksmd-fields.mjs`) for six weeks. This PR lifts the logic into `@tasks-md/lint` so any team can adopt it without copy-pasting a script.

Why opt-in

Most TASKS.md files won't want every task pre-registered — the rule is for teams that want machine-enforced rule-#9 discipline. The flag keeps default behavior unchanged and signals adoption explicitly.

Adopting on an existing repo is usually a two-step ratchet — `--prereg-allowlist=` lets you grandfather legacy task IDs while preventing new tasks from regressing. Grandfathered blocks still surface in the count summary so the operator sees remaining backfill debt without breaking the build.

What

Surface

```bash tasks-lint --require-prereg TASKS.md # enforce the five fields tasks-lint --require-prereg --prereg-allowlist=.prereg-allowlist TASKS.md # ratchet adoption ```

Programmatic API (re-exported from `@tasks-md/lint`):

```ts import { parseRule9Blocks, classifyRule9Blocks, parseAllowlistFile, lintRule9Content } from "@tasks-md/lint"; ```

Output

``` Checked 1 file(s), found 2 error(s) rule-#9 pre-registration: scanned 12 block(s); clean=8, grandfathered=2, blocking=2 ```

Per-block errors:

``` ERROR: TASKS.md: rule-#9 task 'metrics-endpoint' missing Pivot, Anchor; add the field(s) — see https://github.com/tasksmd/tasks.md/blob/main/spec.md#rule-9-pre-registration-block ```

Files changed

• `packages/lint/src/rule9.ts` (new) — pure functions: `parseRule9Blocks`, `classifyRule9Blocks`, `parseAllowlistFile`, `lintRule9Content`
• `packages/lint/src/rule9.test.ts` (new) — 16 tests
• `packages/lint/src/cli.ts` — `--require-prereg` + `--prereg-allowlist=` flag plumbing
• `packages/lint/src/lint.ts` — re-export rule9 module
• `packages/lint/README.md` — opt-in section, summary table row

Test plan

• `npm run build -w packages/lint` — clean
• `npm test -w packages/lint` — 62/62 (16 new + 46 existing)
• `npm test -w packages/parser` — 84/84 (unaffected)
• `npm test -w packages/mcp` — 111/111 (unaffected)
• Default behavior is unchanged when `--require-prereg` is NOT set (one CLI test pins this)
• Allowlist file parsing handles blank lines, comments, inline comments
• Acceptance is treated as equivalent to Success (matches existing spec semantics)

Linked

• spec.md § rule-9-pre-registration-block
• Originating implementation: Minsky `vision.md` § 9

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