Hermes plugin: several named plans without PLAN_ID resolve to the newest plan instead of requiring a selector

Problem

Since v3.17.1 the shell, PowerShell and Python selection paths refuse to pick a plan when two or more named plans exist under .planning/ and PLAN_ID is not set: the prompt-time hook prints [planning-with-files] Multiple plans are available. Set PLAN_ID=<slug> for this session; nothing injected. and every other point stays quiet (#240). A shared .active_plan pointer or a directory timestamp cannot tell which plan a session means, so guessing was removed.

The Hermes plugin (.hermes/plugins/planning-with-files/planning_files.py) still resolves the pointer and then the newest plan by modification time in that situation. Two Hermes sessions working on two named plans in the same project can therefore be injected with each other's plan after a pointer change, which is the shape #240 describes for Codex.

Scope: Hermes users with more than one named plan in one project and no PLAN_ID in the Hermes process environment. One named plan, a root plan, and any PLAN_ID-pinned session are unaffected.

How to reproduce

sh scripts/init-session.sh "Plan A"
sh scripts/init-session.sh "Plan B"      # .active_plan now names plan B
# start a Hermes session in this project without PLAN_ID and send any prompt

Observed: the plan head of Plan B is injected. Expected (Claude Code, Codex, OpenCode and DeepSeek Harness routes): the one-line notice and no plan.

Suggested fix

Port the rule from inject-plan.sh (lines 259 to 277 in v3.19.0): without PLAN_ID, count a root task_plan.md under an armed .planning/sessions/ as one plan plus every valid slug directory that holds task_plan.md; when the count is above one, inject the notice from pre_llm_call once per turn and let post_tool_call, pre_verify, /pwf-status and the tools report that a selector is missing. The OpenCode and DSH plugins received this in v3.20.0 through their shared core.ts (countSelectablePlans), which is a compact reference for the Python port. tests/test_hermes_first_class.py already builds named plans and is the place for the regression.

Origin

Found while porting the rule into the DeepSeek Harness plugin for #252.