Allow adding text sources from files/STDIN in CLI

[K-dash]

Summary

nblm sources add only accepts inline --text arguments. Let users provide longer notes via --text-file path (and optionally --text-stdin) so they can reuse existing documents without copy/paste hassle.

Motivation

• Research notes and transcripts often live in files; pasting them into the command line is error-prone.
• Other CLI tools (e.g. gh, kubectl) support --*-file patterns, making automation easier.
• Implementing this unlocks richer CLI workflows and makes the project more attractive to power users.

Tasks

1. Extend crates/nblm-cli/src/ops/sources.rs to accept --text-file <PATH> (multiple allowed). Read each file, trim it, and push the content as UserContent::Text with an optional --text-name pairing.
2. Optionally add --text-stdin that reads from standard input when present; ensure it can’t be combined with other stdin-consuming flags.
3. Update docs/cli/sources.md with usage examples and note constraints (e.g. empty files should error).
4. Add or update tests in crates/nblm-cli/tests/sources_add.rs to cover the new flags (use temp files in the test helper).

Verification

• Run cargo test --package nblm-cli to ensure CLI tests pass.
• Manual smoke test: cargo run -- sources add --notebook-id demo --text-file sample.txt using a known notebook ID and mock server/base URL.

Recommended for Contributors

• You’ll need to touch both the CLI implementation and documentation, so be comfortable reading Rust and Markdown.
• When reading files, surface friendly errors (e.g. file not found, unreadable). Follow existing error style (use anyhow::Context).