feat: add {file:path} config substitution for reading values from files

[RalphSu2]

Summary

Extends the config substitution system to support {file:path} syntax in addition to the existing ${VAR_NAME} env var substitution. This allows users to reference file contents directly in config values, e.g. for auth tokens:

{
  "mcpServers": {
    "my-server": {
      "url": "https://example.com/mcp",
      "headers": {
        "Authorization": "Bearer {file:~/.ssh/my_token}"
      }
    }
  }
}

Changes

• src/config.ts: Added a second regex pass (/\{file:([^}]+)\}/g) in substituteEnvVars() that reads file contents, trims whitespace, and expands ~ to home directory. Same strict/non-strict error handling as env vars — new MISSING_FILE error type in strict mode, warning + empty string in non-strict mode.
• tests/config.test.ts: 6 new tests covering file substitution, content trimming, tilde expansion, coexistence with ${VAR}, and error handling in both strict and non-strict modes.
• README.md: Updated substitution docs with a syntax table and {file:...} example.

Design decisions

• Synchronous readFileSync — config loads once at startup; consistent with existing existsSync usage. Avoids cascading async refactor.
• Trim file contents to avoid trailing newline issues common with token files.
• ~ expansion via homedir() (already imported).
• No collision between ${...} and {file:...} regexes.
• Order: env vars first, then file refs — allows env var substitution inside file paths.

Test results

bun test tests/config.test.ts
21 pass, 0 fail

[RalphSu2]

fixs #260