Summary

Adds the first #92 context observability slice:

• introduces a core ContextLedger with source kind, persistence class, labels, item count, byte estimate, approximate token estimate, and duplicate keys
• computes the ledger from the final formatted Responses request input, plus base instructions and tool schemas
• logs a compact ledger summary and detailed entries through code_core::context_ledger before Responses requests are sent
• splits project/user instructions from the rendered skills manifest so future /context views can show skill catalog cost separately
• classifies explicit $skill injections as request-only context
• adds fake Responses harness coverage proving the executable path logs ledger categories while the actual request body contains the expected project docs, implicit skills catalog, and explicit manual skill body

Refs #92, #93.

Validation

• cargo test -p code-core client_common::tests::context_ledger
• cargo test -p code-core --test prompt_context_dedup
• python3 tools/code-exec-harness/harness.py tools/code-exec-harness/scenarios/context-ledger-request-summary.json --code-bin /Users/cbusillo/Developer/code/code-rs/target/debug/code
• ./build-fast.sh
• python3 tools/code-exec-harness/harness.py tools/code-exec-harness/scenarios/context-ledger-request-summary.json --code-bin /Users/cbusillo/Developer/code/.code/working/_target-cache/code/feat-context-source-ledger-be2596d583c8-2af88e04f956/code-rs/dev-fast/code