Add --session flag for persistent cookie storage

[ryanfowler]

Implement named sessions that persist cookies across invocations. Sessions are stored as JSON files in the user's cache directory (~/.cache/fetch/sessions/ on Linux, ~/Library/Caches/fetch/sessions/ on macOS). The session name is configurable via --session/-S flag or per-host in the config file.

[Copilot]

Pull request overview

This PR implements persistent cookie storage through named sessions, allowing cookies to be saved and reused across fetch invocations.

Changes:

• Added session management functionality with JSON-based persistence in the user's cache directory
• Integrated session support into the CLI via --session/-S flag and configuration system
• Implemented comprehensive test coverage for session functionality including cookie persistence, expiration handling, and session isolation

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
main.go	Integrated session configuration into the main request flow
internal/session/session.go	Implemented core session management including load/save operations and cookie jar wrapper
internal/session/session_test.go	Added unit tests for session validation, persistence, expiration, and corruption handling
internal/fetch/fetch.go	Integrated session loading/saving into the request lifecycle with error handling
internal/config/config.go	Added session configuration parsing and validation
internal/client/client.go	Added SetJar method to enable cookie jar configuration
internal/cli/app.go	Added --session/-S CLI flag for specifying session names
integration/integration_test.go	Added integration tests for session persistence, expiration, and isolation

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