release: bump ctx to 0.15.0

[luca-ctx]

Summary

• bump public ctx CLI/runtime crates from 0.14.0 to 0.15.0
• update the public CLI artifact build guard and version checks to 0.15.0
• fix clippy issues in the newly merged personal-agent provider import code by using named draft structs for provider event/capture helpers and simplifying a redundant NanoClaw source-format branch

Validation

• bash scripts/check.sh --mode=ci passed 16/16
• scripts/build-public-cli-artifact.sh linux-x64 built ctx 0.15.0 and checked sha256/version sidecars
• local 0.15 smoke passed with isolated HOME and CTX_DATA_ROOT: version, docs search, setup, source discovery, explicit imports, provider searches for Codex/OpenClaw/Hermes/NanoClaw/AstrBot, and read-only SQL