docs: trim internal-monorepo reference from pin rationale

[jeffwall-curlewlabs]

Problem

AGENTS.md Rule #2 (local-mutex pin) included a clause justifying the split floating-vs-immutable pin policy with reference to "Internal monorepo CI workflows (consumed only by us)". That's private-infra detail that doesn't belong in a public repo's contributor docs. Two cross-references in action.yml and save/action.yml pointed at AGENTS.md phrases the trim removes, so they would go stale.

Change

• AGENTS.md: drop the internal-monorepo clause plus the nested "immutable-pin rule applies specifically to dependencies nested inside a published action's own action.yml / save/action.yml" clarifier. Remaining sentences still state the rule, its rationale, and the bump-by-dedicated-PR process without the private context.
• action.yml: comment now says "See AGENTS.md for the rationale" instead of quoting "nested published action dependencies pin to immutable tags" (phrase no longer exists in AGENTS.md).
• save/action.yml: comment now says "See AGENTS.md for the rationale" instead of "for the split-rule rationale" (there is no longer a split rule to refer to).

Invariants preserved

• Pin unchanged (curlewlabs-com/local-mutex@v2.0.0 in both action.yml and save/action.yml).
• Rule unchanged: immutable patch tag, bump via dedicated PR.
• Rationale unchanged: deterministic behavior for local-cache@v3 consumers across reruns.

Verification

• Diff is text-only across three files, no code or CI changes.
• grep -r in the repo turns up zero remaining references to monorepo, curlewlabs2, split-rule, or nested published action dependencies.

Test plan

• CI green