docs: provider pages + pricing overrides reference

Closes the 14-page deficit between code and docs that the provider-
expansion branch had accumulated. Adds one page per provider, all
authored against the actual Go code (provider.go, paths.go, session
readers, widgets, detect dispatch) and modeled on codex.md/opencode.md
for shape and tone.

New provider pages (alphabetical):
amp, codebuff, crush, droid, goose, hermes, kilocode, kimi-cli, kiro,
mux, openclaw, pi, qwen-cli, roocode, zed.

Reference updates:
- providers/index.md bumps the count to 34 and adds tiles for the new
  pages under the existing "Coding agents" section.
- reference/configuration.md gains a "Custom pricing overrides"
  section documenting the custom-pricing.json file: search path,
  format (per-million and per-token spellings), validation rules,
  precedence (custom > LiteLLM > OpenRouter > hardcoded).
- reference/env-vars.md adds OPENUSAGE_CUSTOM_PRICING and
  CODEBUFF_DATA_DIR plus 15 new rows in the CLI / local file
  providers table covering every new local-data provider's primary
  path and override knob.
- sidebars.ts registers the 15 new pages under Coding agents.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>