refactor(config): replace hand-written validation with struct tags

[cpcloud]

Summary

• Replace ~100 lines of hand-written config validation with go-playground/validator/v10 struct tags
• Add custom validators for provider names (validate:"provider"), positive durations (validate:"positive_duration"), and non-negative durations (validate:"nonneg_duration")
• Error messages preserve the existing dotted-path format (e.g. chat.llm.provider: unknown provider "foo")
• Remove deprecated documents.cache_ttl_days field -- cache_ttl has been the canonical setting since its introduction; existing configs using the removed key get an actionable error via checkRemovedKeys
• Update docs and tests to reflect the removal

closes #737

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 92.70833% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.67%. Comparing base (102f15a) to head (0910307).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/config/validate.go	92.22%	6 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
internal/config/config.go	87.90% <100.00%> (-1.82%)	⬇️
internal/config/show.go	74.58% <ø> (-1.67%)	⬇️
internal/config/validate.go	92.22% <92.22%> (ø)

... and 3 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR refactors configuration validation in internal/config from ad-hoc if blocks in LoadFromPath to declarative go-playground/validator/v10 struct tags + a centralized error formatter, and removes the long-deprecated documents.cache_ttl_days setting across code/docs/tests.

Changes:

• Add tag-based validation (validate:"...") for providers, thinking levels, max pages, cache TTL, and OCR confidence threshold, with a new validator setup/formatter in internal/config/validate.go.
• Remove deprecated documents.cache_ttl_days and its related show-config annotations, docs entries, env var mappings, and tests.
• Add validator/v10 dependency and update Nix vendor hash.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/config/validate.go	Introduces validator instance, custom validations, and user-facing error formatting for tag-based validation.
internal/config/config.go	Adds validate: tags, removes cache_ttl_days, and replaces manual validation with cfg.validate(path).
internal/config/show.go	Removes deprecated-path annotation for documents.cache_ttl_days (deprecated map now empty).
internal/config/show_test.go	Drops deprecated cache_ttl_days expectations/tests from ShowConfig coverage.
internal/config/config_test.go	Removes tests and env var mapping entries for deprecated cache_ttl_days.
docs/content/docs/reference/configuration.md	Removes documentation for deprecated MICASA_DOCUMENTS_CACHE_TTL_DAYS / cache_ttl_days.
go.mod / go.sum	Adds github.com/go-playground/validator/v10 and indirect deps.
flake.nix	Updates vendorHash for Nix builds after dependency changes.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.