Add Gajae-Code provider support

[tteggu87]

Summary

• Add Gajae-Code (gjc) as a first-class AI provider next to Claude, Codex, Gemini, and OpenCode.
• Wire GJC into Telegram model selection, execution, session lookup, session archive parsing, and provider availability reporting.
• Document COKAC_GJC_PATH, /model gjc, install guidance, and provider support in README/docs/website docs.

Verification

• cargo test services::gjc_tests --quiet
• cargo check --quiet
• cargo build --release --quiet
• git diff --check
• gjc -p --mode json --no-title --no-session --no-tools @<tempfile> produced text_delta=ok
• cokacctl restart then status/log showed service running with gjc ✓