fix(start-sdk): drop undefined keys when writing an env file model

[MattDHill]

merge(effects, { KEY: undefined }) removes the key from a file model — except on FileHelper.env, which writes the literal KEY=undefined.

What's happening

fileMerge leaves the key present with an undefined value, and z.deepLoose(shape).parse keeps it, so whether it survives to disk has been up to each writer:

writer	drops an undefined-valued key?
json	yes — JSON.stringify omits it
yaml / toml / xml	yes — their serializers omit it
ini	yes — INI.stringify(filterUndefined(inData)), explicitly
env	no — Object.entries(inData).map(([k, v]) => \${k}=${v}`)` stringifies it to the word

Same shape, same file, merge(effects, { K: undefined }) on 2.0.9:

json  -> {"A":"keep"}                 removed
ini   -> A=keep                       removed
yaml  -> A: keep                      removed
toml  -> A = "keep"                   removed
xml   -> <root><A>keep</A></root>     removed
env   -> A=keep\nK=undefined          NOT removed

A key simply absent from the merge object is a correct no-op in all six.

The fix

Filter in the one place write and merge both render through, rather than per writer — the contract then holds for every format instead of five of six by accident, and merge's dirty-check compares the same bytes it goes on to write. The ini writer's own filterUndefined call becomes redundant and is removed.

Why it matters

FileHelper.env is rare in the fleet — three packages across both registries — which is why this held up unnoticed. One of them hits it: bitcoin-explorer-startos writes

BTCEXP_REDIS_URL: input.redis ? redisUrl : undefined

so turning Redis off in its Configure action writes BTCEXP_REDIS_URL=undefined. btc-rpc-explorer then gets a Redis URL of "undefined", while the shape's .catch(redisUrl) masks the bad value on read — so the package's own guard still reports Redis as enabled and the setting silently doesn't take. Filed as an issue on that repo.

Tests

lib/test/fileHelper.merge.test.ts covers both directions across the five text formats — a key merged as undefined is removed, a key the merge doesn't name is left alone. Verified it fails on the env case without the fix (1 failed, 9 passed) and passes with it.

Full suite on this branch: 9 passed, 117 tests.