fix: resolve cortex_skill dir with flags.PROJECT_DIR for dbt Fusion

[mattsenicksigma]

Closes #32.

Problem

cortex_skill__resolve_skill_dir() (in macros/relations/cortex_skill/create.sql) returns a path built from model['original_file_path'], which is relative to the project root. dbt Core sets its process cwd to the project root when executing the PUT, so the relative path resolves fine there — but dbt Fusion does not, so the PUT fails with:

[Snowflake] 264006: file does not exist: [models/agents/agents_schema_skill/*]

Fix

Build an absolute path from flags.PROJECT_DIR ~ '/' ~ model['original_file_path'][:-4] instead. flags.PROJECT_DIR is populated in both dbt Core and dbt Fusion, whereas model['root_path'] resolves to an empty string under Fusion (per the issue report), so it isn't a viable alternative.

Also updated the doc comments in the same file that described the old relative-path behavior.

Test plan

• Reviewed the change compiles the same PUT statement shape, just with an absolute local path prefix
• Matches the fix the issue reporter said they tested locally and confirmed resolves the problem
• Repo has no unit tests for this macro; the existing integration_tests CI (DuckDB compile of agent_*/example_mcp_* models) doesn't currently select the skill fixture, so this fix isn't independently re-verified by CI — flagging for the maintainer to confirm against a real Fusion run if desired

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