refactor!: remove header CLI and REPL support

[andylokandy]

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[Copilot]

Pull request overview

This PR removes the ability to set/manage custom HTTP headers via the CLI (--header) and via the REPL (/headers), consolidating header configuration to connection configuration (including env-based config overrides). It updates the internal client construction to accept extra headers at creation time instead of mutating them after.

Changes:

• Remove --header from CLI argument structs and eliminate runtime parsing/propagation of CLI headers.
• Remove the REPL /headers subcommand and associated header-mutation plumbing.
• Refactor ScopeQLClient/Client construction to carry extra_headers immutably, and add coverage to verify extra headers are included in request headers.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scopeql/src/repl/entrypoint.rs	Drops /headers handling and removes headers parameter from REPL entrypoint.
scopeql/src/repl/command.rs	Removes the /headers subcommand definitions from the REPL command parser.
scopeql/src/main.rs	Removes CLI header parsing and stops passing headers into REPL/run/load code paths.
scopeql/src/load.rs	Removes HeaderMap parameter and relies on connection-config headers via ScopeQLClient::from_connection.
scopeql/src/execute.rs	Removes HeaderMap parameter and relies on connection-config headers via ScopeQLClient::from_connection.
scopeql/src/command.rs	Removes --header options from ReplArgs and ExecArgs.
scopeql/src/client/mod.rs	Removes mutable header APIs and passes parsed connection headers into Client::new(...).
scopeql/src/client/connection.rs	Extends Client::new(...) to accept extra headers; ensures request headers include them; updates/adds tests.
scopeql/src/header.rs	Updates doc comment to reflect broader usage of parse_headers.
CHANGELOG.md	Documents the breaking removal of CLI/REPL header management and points users to config/env-based headers.

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