feat(langchain): add LC-021, LC-022 TypeScript description quality rules

[bradAGI]

The TypeScript counterpart to LC-018/019 (#66). LC-010 only checks that a description exists, so a tool described "TODO: describe this tool." or "Gets data." passes today while giving the model no selection signal.

Three points, in the order they bite:

• No docstring fallback. LangChain.js has nothing behind the description field, so it's the entire account of the tool the model sees — unlike the Python side, where a thin description at least sits next to readable source.
• The Zod schema doesn't compensate. It constrains the shape of the arguments once the model has chosen this tool, never whether choosing it was right.
• Mis-selection scales badly and costs the run. A LangChain agent is routinely handed a dozen or more tools at once, and each wrong pick spends an iteration against the maxIterations bound LC-111 checks for — so a run can exhaust its step budget and return nothing useful.

Verification — engine built at main:

$ trustabl rules validate .
OK: 85 rule pack(s), 208 rule(s) valid under rule schema version 14

Fire (one tool(...) described "TODO: describe this tool.", one "Gets data."): LC-021, LC-022, LC-201
Silent (full description naming when to prefer the neighboring tool): LC-201

(LC-201 is the pre-existing missing-AGENTS.md repo rule.)

LC-022 pairs description_length_lt: 40 with has_docstring: true so it doesn't double-report against LC-010 on an empty description, same as CSDK-018.

No new predicates, so no schema_version bump.

Merge note: this and #66 both append to langchain/tool_definition.yaml, so whichever lands second needs a trivial rebase — the rule blocks are independent and the resolution is "keep both." Happy to rebase on request.

[jhumel-code]

Thanks @bradAGI, this is a clean one. Checking both rules against origin/main: has_description_text, has_docstring and description_length_lt all exist in internal/rules/schema.go, langchain_tool is a valid applies_to at scope: tool, and LC-021 / LC-022 are free on main and unclaimed by any other open PR. Severity and confidence line up with LC-001 and LC-010 next door, and pairing description_length_lt: 40 with has_docstring: true so that an absent description stays LC-010's finding rather than double reporting is exactly right, the same shape as CSDK-018. No new predicates, so you are correct that no schema_version bump is needed here.

The engine half in trustabl/trustabl#171 mirrors this file byte for byte (both sides land on the same blob hash), so rules-sync stays green as long as the two land together.

One thought rather than a blocker: todo is matched as a case-insensitive substring, so a genuine tool described "Add an item to the user's todo list." will fire LC-021. CSDK-017 already ships that needle, so I am not asking you to diverge from it here.

Before this ships it needs a rationale doc in trustabl-rulebook for the LangChain pack. That part is on us, and we are happy to pair on it. Looks good to me otherwise.