Add `validate` and `check` CLI commands

[dgenio]

Context

Split from #33 (CLI meta-issue). This issue adds flow validation commands to the CLI established in #44.

What to do

1. validate command

chainweaver validate <file> validates a flow definition file (YAML/JSON):

chainweaver validate flows/my_flow.flow.yaml

• Loads the flow from file using serialization from Implement flow serialization and deserialization (YAML + JSON) #14
• Validates structure (required fields, valid step references)
• Reports errors with file, line context
• Exit code: 0 = valid, 1 = validation errors, 2 = file not found

2. check command

chainweaver check <directory> validates all .flow.yaml / .flow.json files in a directory:

chainweaver check flows/

• Scans directory for flow files
• Validates each one
• Summary output: N valid, M invalid
• --quiet flag for CI (exit code only)

3. Output formatting

• --format json for machine-readable output (list of validation results)
• --format table for human-readable table (default)

Files to modify

• chainweaver/cli.py — add validate and check subcommands
• tests/test_cli.py — add validation command tests

Acceptance Criteria

• chainweaver validate <file> validates a flow file and reports errors
• chainweaver check <dir> validates all flow files in a directory
• Invalid flows produce clear error messages with details
• Exit codes: 0 = all valid, 1 = validation errors, 2 = file/dir not found
• --format json produces machine-readable output
• --quiet suppresses output (exit code only)
• At least 4 test cases: validate valid flow, validate invalid flow, check directory, file not found

Out of Scope

• inspect command (→ Implement CLI entry point with inspect command #44)
• viz command (→ separate issue)
• Schema chain compatibility checking against registered tools (requires executor context)

Dependencies

• Implement CLI entry point with inspect command #44 (CLI entry point must exist first)
• Implement flow serialization and deserialization (YAML + JSON) #14 (flow serialization — needed to load flow files from disk)
• Add compile-time flow validation (tool existence + schema chain compatibility) #7 (compile-time validation — the validation logic itself)

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Part of CLI split from #33. See also: #44 (CLI entry + inspect), viz CLI.