[STEP-2562] Report what the parser finds in xcodebuild_options

[lpusok]

Every builder now returns Command.Diagnostics: findings about the step's additional
options, each ending with the fix ("Remove it." or "Use instead.").
This commit adds the framework and the parser's findings:

• MalformedOption: an argument xcodebuild refuses (an empty argument, "-sdk macosx"
  quoted as one, a lone dash, "-only-testing:" with no value, a bare word from an unquoted
  value with spaces, a value flag at the end);
• SuspiciousUserDefault: an option xcodebuild silently ignores ("-destination
  generic/platform=iOS" quoted as one argument, -ENABLE_BITCODE=NO for a build setting).
  The message says today's build runs without it, and offers removing it or the corrected
  form, which applies it.

Corrected forms are rendered with go-shellquote's Join, the inverse of the step's split.
Options.Diagnostics gives a step the parser's findings before its first xcodebuild call.

Validation decides what a builder does with findings: Warn (default) passes everything
through and reports; Fail returns the first one that is not informational as an error.
Nothing changes on the command line.

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

Corrective messages mishandle tab-separated flags, and the package documentation overstates action-level validation.

2 open findings

• Split combined flags at any whitespace boundary · New
• Clarify validation scope excludes action-incompatible options · New

What changed in this PR

Adds parser diagnostics for malformed or suspicious xcodebuild_options, with configurable warning or failure behavior.

Changes:

• Adds diagnostics and validation APIs to assembled commands.
• Detects malformed options and suspicious user defaults with corrective messages.
• Adds diagnostic and validation tests while suppressing duplicate package-resolution diagnostics.

File	Description
xcodecommand/​xcodecommand.go	Updates package documentation.
xcodecommand/​resolve_package_deps.go	Documents diagnostic suppression.
xcodecommand/​resolve_package_deps_test.go	Verifies diagnostics remain empty.
xcodecommand/​options.go	Documents parser diagnostics.
xcodecommand/​options_test.go	Tests diagnostic classification.
xcodecommand/​export.go	Adds export validation configuration.
xcodecommand/​diagnostic.go	Implements diagnostics and validation.
xcodecommand/​diagnostic_test.go	Tests messages, validation, and quoting.
xcodecommand/​command.go	Stores diagnostics and enforces validation.
xcodecommand/​build.go	Adds validation to build actions.
xcodecommand/​archive.go	Adds archive validation configuration.

🧠 Review effort: Balanced

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