feat: add opt-in capture conditioning flags

[rossarmstrong]

• Cargo.toml: enable decibri's gain feature (no new dependencies, so Cargo.lock is unchanged; default-features stays off so no vad/denoise/ort are pulled)
• src/commands/capture.rs: add --dc-removal, --highpass , --agc , --limiter as opt-in flags wired to MicrophoneConfig; each is an independent optional flag so future stages are additive. The high-pass cutoff validates to the library's closed set (80, 100), matching the Python binding.
• capture --json: add an always-present nested conditioning object reporting the active stages
• README.md: document the four flags, a clean-ASR-input example, and the corrected trimmed feature set (capture, playback, gain)
• CLAUDE.md: update the decibri feature-set fact and record the conditioning wiring
• CHANGELOG.md: Unreleased Added entries
• tests: capture help flag list, exit-2 validation coverage, and parser/serialization unit tests