Load operator extension paths in atomic Planner sessions

HARNESS_EXTENSIONS lists extension or package paths that every atomic
Planner session loads, as Atomic's --extension flag would. A package's
extensions, skills and workflows all register, so an operator can give the
Planner workflows without installing them into the agent directory. The
paths are split on the platform path delimiter, must be absolute and exist,
and are refused under any other harness. Background workers never load
them, and the startup line names them.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-verification: bun test passed: 1907 pass, 2 PostgreSQL skips, 0 fail; the new Planner test fails without the adapter change
Assistant-verification: bun run types passed: all workspaces
Assistant-verification: bun run ci passed: dprint, oxlint (7 existing warnings, none new), tokens, design contract, design record, Impeccable (no new findings)
Assistant-verification: adapter integration passed: a Planner session with a local package on HARNESS_EXTENSIONS listed that package's workflows through the workflow tool
Co-authored-by: Alex Lavaee <lavaman131@github.com>