mdcode: address KC pull review comments (post-merge follow-up)

[libei]

Follow-up to the merged #277 (semantic-model Knowledge Catalog pull leg), addressing the inline review comments left after merge. Reader / pull-orchestration / CLI only — the writer (#278) is untouched.

Changes

• pull_kc: hard-fail on any entry-hydration fetch error and on a non-200 link lookup; an empty link list stays a silent no-op (:81, :112).
• One model per entry group: dropped the pull --model flag; >1 anchor in a group is now a hard error naming both (:483, :261, :140).
• pull --force-remove: authorize replacing a differently-named local model with the catalog's, rather than silently leaving two models in the group (:269).
• Schema-join link type is a fixed built-in constant, never undefined (:108, :182).
• push: warn when a relationship name will be normalized — KC stores the name only in the link id, so a pull returns it lowercased/hyphenated (:295).
• Typeless defaults: untyped fields and metrics now default to Opaque (STRING + OTHER) instead of guessing STRING/NUMERIC; a typeless metric round-trips un-typed (:296).
• osi_schema guardrail: the .pull.golden.yaml fixtures come from a default (expression-free) KC push, so their fields/metrics lack the OSI-required expression. They're excluded from the OSI schema check with a TODO(#290) — mdcode: store SQL expressions in the sql-expressions companion aspect #290 (the sql-expressions companion aspect) restores expressions on push+pull, at which point they can be schema-checked again.
• Docs (docs/semantic-model.md) + tests + regenerated goldens to match.

Deferred (tracked as follow-ups, not in this PR)

• Recover entity keys (pk/unique) on pull → schema aspect.
• Recover ai_context on pull → guidelines aspect.
• Recover field labels on pull → schema aspect annotations.

Testing

bun test — green (the one local ajv/dist/2020 load error is a borrowed-node_modules artifact; ajv ^8.17.1 is a declared devDependency so CI resolves it).

[libei]

Follow-up commit b2ee552 from a self-review pass:

• Pull reconcile compares by on-disk path, not raw name. A model name that sanitizes to the same file (e.g. a/b → a_b.yaml) was flagged as a stale conflict on every re-pull because catalogNames held the raw name while modelDocuments() returns filename-derived names. Now both sides are compared through modelPath.
• mapConcurrent stops fanning out after the first failure. Promise.all already rejects on the first throw, so the remaining workers' fetches were wasted work whose rejections surfaced as unhandled-rejection noise; a shared failed flag now halts new claims.
• OSI guardrail validates the .pull.golden.yaml fixtures too, tolerating only the known missing-expression gap (TODO mdcode: store SQL expressions in the sql-expressions companion aspect #290) rather than skipping them by filename — so unrelated schema drift is caught today, and the fixtures schema-check with no special-casing once mdcode: store SQL expressions in the sql-expressions companion aspect #290 restores expressions (verified: all three fail only with required:expression).

Deferred, intentionally not in this commit:

• FK-direction orientation on live pulls relies on dataSource string equality, which is not normalization-robust the way endpoint identity (matched by entry id) is. If Dataplex normalizes an entity aspect's source resource but not the link references, direction could fall back to reference order (warned, not errored). Needs live-KC confirmation and is closer to writer-side (mdcode: KC push follow-ups (validate gate, model + link reconciliation) #278/mdcode: store SQL expressions in the sql-expressions companion aspect #290) scope.
• --dry-run returning a non-zero exit on a stale-local conflict — arguably correct (it signals the real run would fail); left as-is.

tsc clean; full suite 330 pass / 0 fail.