fix: surface validation issues in update and delete error output

[chrisstrahl]

Problem

When collection.update() rejects a write, the library returns both result.error and result.issues (an array of {code, message, field, severity}), but the update command prints only error.message ("Validation failed on update") and discards the issues in both text and --format json output.

That makes validation failures undiagnosable from the CLI — you can't see which field failed or why. In practice this hid a real library bug for a while (see callumalpass/mdbase#3, where a generated date field caused every update to fail with nothing but the one-line message).

Before:

$ mdbase update hello.md -f rating=10
error: Validation failed on update

After:

$ mdbase update hello.md -f rating=10
error: Validation failed on update
  error field rating: Integer "rating" is too large (10 > 5) [number_too_large]

and in JSON:

{
  "error": { "code": "validation_failed", "message": "Validation failed on update" },
  "issues": [
    { "code": "number_too_large", "message": "Integer \"rating\" is too large (10 > 5)", "field": "rating", "severity": "error" }
  ]
}

Changes

• update: include issues in JSON error output and print them as indented lines in text output — same format create already uses.
• delete: same passthrough for output-contract parity. (No library code path returns issues on delete today, so this is defensive; happy to drop it if you'd rather not carry the dead branch.)
• utils.ts: formatIssue moved here from create.ts so all three commands share it. No behavior change for create.
• Tests: new test/fixtures/strict-collection fixture with default_validation: error (the default warn level never rejects updates, so the existing fixture can't trigger this path). Two new update tests (JSON + text), plus regression tests pinning create's existing issue output. Fixture's .mdbase/ cache dir is gitignored.

Testing

• New update tests written first and confirmed failing against the old code (issues absent from both formats), then passing with the fix.
• Full suite: 269/269 passing, tsc clean.

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