docs(sampling): clarify that sampler override presets resolve from the working directory (#419)

[Anai-Guo]

Fixes #419.

Problem

sampler_override / override_preset takes a preset name, which overrides_from_file() turns into the relative path sampler_overrides/<name>.yml. A relative path resolves against the process's current working directory, so the folder that actually gets searched is the one you launched TabbyAPI from — not the one next to the TabbyAPI source.

Nothing in the docstrings, the config description, config_sample.yml, or the docs said this, and the failure message did not help either:

Sampler override file named "X" was not found. Make sure it's located in the sampler_overrides folder.

Which sampler_overrides folder? The one shipped in the repo has the preset in it, so from the user's side this reads as a bug rather than a working-directory mismatch (see #419 and turboderp-org/exllamav3#199).

Change

No behaviour change — this is the "adjust the docstring to make it clear how the search happens" option from the issue.

• Hoist the lookup directory into a SAMPLER_OVERRIDES_DIR constant with a comment explaining the relative-path semantics, and use it from both overrides_from_file() and get_all_presets() (they previously spelled "sampler_overrides" out separately).
• Make the not-found error name the resolved path it tried and the current working directory, so the mismatch is visible from the message alone.
• State in the two docstrings, the override_preset config description, config_sample.yml, and docs/02 + docs/08 that the value is a preset name rather than a path, resolved relative to the launch directory.

While there, the config description and config_sample.yml both said "sampler-overrides folder"; the folder is sampler_overrides.

The issue also floated dropping the name→path transformation and accepting a full path instead. I left that alone since it changes existing configs — happy to follow up if you'd prefer that direction.

Verification

ruff format --diff and ruff check clean with the pinned ruff == 0.11.10 against the repo's pyproject.toml.

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