fix: enforce fresh context between molecule steps

Change molecule step completion instructions to use `gt mol step done`
instead of `bd close`. This ensures polecats get fresh context between
each step, which is critical for multi-step review workflows like
shiny-enterprise where each refinement pass should have unbiased attention.

The `gt mol step done` command already:
1. Closes the step
2. Finds the next ready step
3. Respawns the pane for fresh context

But polecats were being instructed to use `bd close` directly, which
skipped the respawn and let them run through entire workflows in a
single session with accumulated context.

Updated:
- prime_molecule.go: step completion instructions
- mol-polecat-work.formula.toml
- mol-polecat-code-review.formula.toml
- mol-polecat-review-pr.formula.toml

Fixes: hq-0kx7ra

internal/formula/formulas/mol-polecat-work.formula.toml

@@ -9,10 +9,13 @@ crash after any step and resume from the last completed step.
 
 You are a self-cleaning worker. You:
 1. Receive work via your hook (pinned molecule + issue)
-2. Work through molecule steps using `bd ready` / `bd close <step>`
+2. Work through molecule steps using `bd ready` / `gt mol step done <step>`
 3. Complete and self-clean via `gt done` (submit + nuke yourself)
 4. You are GONE - Refinery merges from MQ
 
+**Fresh context:** Each `gt mol step done` respawns your session with fresh context.
+This ensures each step gets unbiased attention.
+
 **Self-cleaning:** When you run `gt done`, you push your work, submit to MQ,
 nuke your sandbox, and exit. There is no idle state. Done means gone.