fix(diff): emit the unresolved_target slug, not the sentence

[KageBinary]

docs/llm-format.md:29 says a target that does not exist is always unresolved_target, whichever command was asked. #538 unified everything except diff.ts, and nothing has touched it since.

Found while reviewing #547: twelve commands resolve a target, #547 converts ten, #559 takes locate, and diff is the twelfth that nobody has.

What was wrong

ix diff --format json on a target that does not exist emitted:

{"error": "No entity found matching \"decide.ts\"."}

The human sentence sits in the error field — where every other command puts the machine slug — and there is no message field at all. A consumer routing on error === "unresolved_target" gets an English sentence; one reading message gets nothing.

There are two such sites, not one. The symbol path (resolveEntityFull, diff.ts:514) and the file-like path (resolveFileOrEntity, diff.ts:520) each carry their own copy, so ix diff 3 5 Foo and ix diff 3 5 Foo.ts were separately broken. Both are fixed and separately tested.

The --format llm branch already emitted the right slug, but with the older No entity resolved for "…" wording. That is now the shared text, so the string is gone from diff.ts. The eight remaining copies belong to the commands #547 converts and are deliberately left alone to avoid conflicting with that PR.

unresolvedTargetMessage and unresolvedTargetRecord are split out of reportUnresolvedTarget so a command can emit the shared record without the exit code — which is the whole point of this PR being separable.

The exit code is deliberately not in this PR

Making ix diff exit non-zero is a breaking change for the plugins (CONTRIBUTING.md → CLI Standards → Exit codes). ix-cursor-plugin's ix_diff tool routes on the exit status at mcp/tools/map.ts:206; today it reads raw.summary/raw.total off the error object and reports a successful "0 changes" for a target that does not exist. Only the exit code fixes that, and it belongs in #547's queue behind the five plugin PRs.

A test pins the current exit behaviour so nobody adds it without doing that work.

Tests

Four, each mutation-checked against the code it covers:

mutation	result
revert both json sites to the prose-in-error shape	2 failed / 2 passed
revert the llm wording only	1 failed / 3 passed
add process.exitCode = 1	1 failed / 3 passed

Verification

typecheck, eslint and knip clean; ix-cli 1509 passed / 2 skipped.

One thing worth flagging separately: src/cli/__tests__/watch-dedup.test.ts ("starts both the tsx source CLI and a normal built CLI child") fails when run in isolation on unmodified main — 4 of 4 attempts at 793c56b with this change stashed. It passes in the full suite, so it is order- or build-dependent rather than broken. Pre-existing and unrelated to this PR, but it should probably get an issue.