feat(ctx): phase-2 chain — one-hop reference + supersession neighborhood

[eris-ths]

What

The second phase-2 ctx verb (read-side): ctx chain <id>.

Shows the records adjacent to a fact in one hop, so a reader can follow related observations without grepping the substrate — the ctx analogue of gate chain.

Edge kinds (all from a single substrate scan)

• outbound — ctx ids the root's own prose mentions. Resolved to their facts; a prose mention of an absent id is surfaced as (referenced but not found) rather than dropped (records-outlive-writers). Self-references ignored.
• inbound — other facts whose prose mentions the root id.
• supersedes — the fact the root corrects (its forward link).
• superseded by — the facts that correct the root (a fork yields more than one).

One hop only, like gate chain: to go deeper, run ctx chain on a surfaced id — the reader drives the depth. A missing root is a recoverable not-found; an isolated fact reports an empty neighborhood rather than an error.

Shared scanner change (affects gate chain too — behavior unchanged)

extractReferences (the lexical id-scanner behind gate chain and UnrespondedConcernsQuery) gains a third id kind: ctx-YYYY-MM-DD-NNN, returned in a new ctxIds field. This also fixes a latent mis-classification: the leading (?<!\w) boundary allowed a hyphen, so a ctx-… (or i-…) id's digits could leak into requestIds. Capturing the prefix in the pattern keeps the three kinds disjoint. gate chain does not surface ctx ids, so its behavior is unchanged — covered by the existing chain regression tests plus new direct unit tests for the scanner.

Verification

• Full suite green: 1859/1859 pass, 0 fail (node tests/run.mjs, exit 0).
• Dogfooded through the real binary: supersede-chain both directions, inbound prose reference, outbound + dangling ref, one-hop (no transitive walk), empty neighborhood, error paths.
• New tests: chain.test.ts (7 e2e) + extractReferences.test.ts (6 unit, incl. the mis-classification regression guard).
• Docs (verbs.md / AGENT.md / playbook.md / CLAUDE.md) + changelog fragment updated.

Remaining phase-2

fork / status.

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