feat: add --context flag to set model context window - #2

Add a --context/-c flag to oc-config add for setting a model's context window, with forgiving parsing of human-friendly sizes.

Summary

• New --context/-c flag on oc-config add writes each added model's window as limit.context in the opencode config.
• Lenient parsing: decimal suffixes (k/m/g/t), absolute counts (200000), fractions (1.5m), and stray commas, underscores, whitespace, or a trailing tokens word all parse as you'd expect.
• Existing limit keys (e.g. output) are preserved when context is merged in.
• --providers loses its -c short alias (long form only) so -c can serve --context.
• README and both llama.cpp guides document the flag; the guides show matching opencode's window to the server's --ctx-size.
• Unit tests for the parser, the applier, and the CLI path; total coverage rises from ~77% to ~89%.

Implementation details

Suffixes are decimal (128k = 128,000 tokens), which matches how context windows are usually quoted, rather than binary (1024). Context is applied to every model added in the invocation by merging a limit map onto each entry, so it composes with the existing deep-merge write path without clobbering sibling limit keys.