Add project configuration and document-glob support

[adrianbrowning]

What to build

Add a project-level mdcode configuration that defines the Markdown documents, source roots, filters, and validation defaults for a repository. Consumers should be able to run common workflows without repeating command-line wiring in every local script and CI job.

Acceptance criteria

• A documented configuration format supports document paths or globs, source/output roots, and default filters.
• The CLI can load the configuration for check and synchronization workflows.
• Command-line options can intentionally override configuration defaults.
• Invalid configuration produces actionable validation errors.
• A complete example is included for a consumer repository and CI job.

Blocked by

• Add plan, diff, apply, and check workflows for agent-driven sync #6

[adrianbrowning]

This was generated by AI during triage.

Agent Brief

Category: enhancement
Summary: Add a small, non-executable project configuration for mdcode documents, roots, and default filters.

Current behavior:
Every workflow must repeat document paths, working roots, and filters on the command line. There is no mdcode configuration convention or configuration validation.

Desired behavior:
A repository can define common defaults in mdcode.config.json. The CLI discovers that file from the working directory or accepts an explicit --config path. Commands use configuration values as defaults, while explicit CLI options intentionally override them.

Key interfaces:

• Configuration file — JSON only, named mdcode.config.json. Do not execute JavaScript or TypeScript configuration.
• Configuration shape — documents is an array of Markdown paths or globs; sourceRoot and outputRoot define default operation roots; filter supplies default language and metadata filtering.
• Path resolution — resolve configuration-relative paths against the configuration file directory and apply existing guarded-path rules to file operations.
• CLI precedence — explicit command-line document inputs, roots, and filters override the corresponding configuration defaults.
• Validation — reject malformed JSON, unknown/invalid field shapes, unmatched required document selections, and unsafe paths with actionable messages.

Acceptance criteria:

• mdcode.config.json supports document paths or globs, sourceRoot, outputRoot, and default filters.
• The CLI discovers the default configuration or loads an explicit --config path for check and synchronization workflows.
• Explicit CLI options override configuration defaults predictably.
• Configuration paths are resolved relative to the configuration file and unsafe paths fail clearly.
• Invalid configuration produces actionable validation errors.
• Documentation includes a complete consumer configuration, local command, and CI example.
• Tests cover discovery, explicit selection, glob expansion, precedence, and validation errors.

Out of scope:

• Executable JavaScript or TypeScript configuration.
• Watch-mode behavior, which is issue Add a safe watch mode for local documentation development #18.
• Redefining plan, apply, diff, or check semantics from issue Add plan, diff, apply, and check workflows for agent-driven sync #6.
• A multi-project monorepo configuration model beyond paths/globs that work from one config file.

Verification status:
Confirmed: no existing mdcode configuration convention exists. This issue remains blocked by #6.