feat(talents): add cwd frontmatter, default cogitate agents to journal
Adds an optional `cwd` field on talent frontmatter with two legal values (`"journal"` default, `"repo"` escape hatch) for `type: cogitate` prompts. Threads the resolved absolute path through `prepare_config` and each provider's `CLIRunner`, and also through the cortex-side `Popen` for `sol agents`, so every cogitate subprocess lands in the correct working directory. Validation is centralized in `_validate_cwd` (`think/talent.py`) and runs at both `get_talent_configs` and `get_agent`. `talent/coder.md` opts into `cwd: "repo"`; all other cogitate talents default to `"journal"`. Generators reject `cwd` entirely. Fixes repo-relative path notes in `talent/heartbeat.md` so the agent runs correctly under journal cwd. Known gap: Google provider's `--sandbox=none` is unchanged and deferred to a follow-up lode.