Align capability bundle terminology

[DemonGiggle]

Summary

• rename the public capability-bundle API from ec_skill_* to ec_capability_* terminology
• rename implementation files to ec_capability.c and ec_capability_table.c
• remove the unreleased ec_skill.h compatibility shim so the codebase uses the correct terminology cleanly
• update docs, comments, tests, and config naming so model-callable functions are "tools" and compile-time tool/context/policy groups are "capability bundles"

Verification

• cmake -S . -B build-posix -DEC_PLATFORM=POSIX -DEC_ENABLE_TLS=OFF
• cmake --build build-posix
• ctest --test-dir build-posix --output-on-failure
• cmake -S . -B build-host-sim -DEC_PLATFORM=POSIX -DEC_HOST_SIM=ON -DEC_ENABLE_TLS=OFF
• cmake --build build-host-sim
• ctest --test-dir build-host-sim --output-on-failure
• cmake -S . -B build-baremetal -DEC_PLATFORM=BAREMETAL -DEC_ENABLE_TLS=OFF
• cmake --build build-baremetal
• cmake -S . -B build-baremetal-tls -DEC_PLATFORM=BAREMETAL -DEC_ENABLE_TLS=ON
• cmake --build build-baremetal-tls

[gemini-code-assist]

Code Review

This pull request performs a comprehensive renaming of the 'Skill' system to 'Capability Bundles' throughout the codebase, including source files, headers, build configurations, and documentation. It introduces the new capability framework while maintaining a compatibility shim in ec_skill.h for existing integrations. The review feedback suggests improving the robustness of the initialization process by having ec_capability_init return an error code to signal failures such as tool registration overflows or system prompt buffer truncations.

[gemini-code-assist]

The ec_capability_init function currently returns void, but it performs several operations that could fail, such as registering tools (which can fail if the registry is full) and building the system prompt (which can fail if the buffer overflows). Consider returning an error code (e.g., int) to allow the caller to handle initialization failures gracefully, especially in an embedded context where silent truncation of the system prompt or missing tools can lead to confusing behavior.