docs: parent_id auto-discovery — executable lookup before save

[rmichaelthomas]

Fixes the gap observed live this session: an agent inheriting from a prior contract that was saved to Receipts still omitted parent_id, because the "Inheritance and lineage" section (PR #7) documented what and when but not how to find it. The lineage feature shipped dead — API supports it, frontend renders it, but the skill never performed the lookup.

Changes

• Replaces the passive "how to obtain the parent ID" bullet list with an executable discovery procedure: query GET /api/v1/export, parse the contract history, match the prior contract by label or recency, and use its ID as parent_id. Includes a fallback to save the prior contract first (capturing its returned ID) if it was never saved. Tier 1 fallback (extract ID from a known permalink) preserved.
• Adds a "resolve parent_id before saving" preamble to the session-end save step (step 2), cross-referencing the discovery procedure and stating that skipping it breaks the lineage chain.
• Broadens the documented classifier allow rule from …/save only to "Bash(curl -s * https://receipts.liminate.dev/* *)" so the new GET /api/v1/export call works in auto-mode (failure mode feat: session pack v1 — 3 nouns, 2 verbs (cite, verify) #1). Tightly-scoped two-rule alternative noted.

Invariants

• Discovery and save procedures cross-reference each other; neither assumes the other was read.
• Export call uses $RECEIPTS_API_KEY — no new credentials.
• Discovery is best-effort, not blocking: if export fails or returns nothing, the agent omits parent_id and saves anyway.
• One file changed (SKILL.md).

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