Sync unified docs with in-process swift-syntax-rs parsing (post-#22290)

[Copilot]

PR #22290 dropped the separate swift-syntax-parse binary in favor of calling swift_syntax_rs::parse_to_json in-process, but explicitly left the .md docs unupdated. unified/AGENTS.md and unified/swift-syntax-rs/README.md still described the old out-of-process binary workflow.

unified/AGENTS.md

• "Swift Parser" section now describes in-process parsing via swift_syntax_rs::parse_to_json, no separate binary or grammar to edit.
• Testing bullet drops the CODEQL_EXTRACTOR_UNIFIED_SWIFT_SYNTAX_PARSE env var and corpus-test-skip guidance; now notes cargo test needs a working Swift toolchain (since swift-syntax-rs builds in-process), and that tests are also runnable via bazel test //unified/extractor:all_tests.

unified/swift-syntax-rs/README.md

• Prerequisites: removed the SWIFT env var mention — build.rs no longer reads SWIFT/SWIFTC, just requires swift/swiftc on PATH.
• Building & testing: removed the SWIFT=… SWIFTC=… cargo build fallback block.
• Building with Bazel: clarified swift-syntax-parse is a debugging aid, not shipped in the extractor pack.
• Converting to a yeast AST: corrected to say the extractor links swift-syntax-rs directly and consumes JSON produced in-process, replacing the stale "out-of-process ... binary" framing.

Documentation-only change; no code, build, or CI behavior affected.

[Copilot]

Pull request overview

Updates unified Swift documentation to reflect in-process swift-syntax-rs parsing.

Changes:

• Documents current Swift toolchain requirements.
• Clarifies Bazel/debug binary usage and extractor integration.
• Updates extractor testing instructions.

Show a summary per file

File	Description
unified/AGENTS.md	Updates parser architecture and testing guidance.
unified/swift-syntax-rs/README.md	Revises prerequisites, build instructions, and integration details.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Balanced