feat(dev): self-activating scripts with a stale-aware activation sentinel

Repo scripts assume the PM-activated environment, so running one without
activation fails much later with a confusing ImportError. Add the two halves
covering both invocation paths:

- scripts/_activation.py: require_activation() exits immediately, naming the
  exact command for the caller's shell (source ./activate on POSIX,
  . .\activate.ps1 on a native Windows host), before any heavy import.
- scripts/_hermes-python: the POSIX shebang target. `#!/usr/bin/env -S bash -c
  '...'` hands itself the target path through bash -c's $0, sources activate,
  then execs the interpreter on the same file -- so tracebacks and __file__
  still point at the real script and ./scripts/foo.py works from any cwd with
  no manual source.

__HERMES_ACTIVATED changes from a bare "1" to the installed-state file the
environment was composed against, so one value carries activation, which
checkout activated it, and a staleness stamp. The prologue compares that file
against uv.lock / pyproject.toml / pm/lock.json with the `-nt` builtin -- no
process spawn -- and re-activates once when the inherited environment predates
its inputs. pm rewrites that file only on a real sync, so the check settles
back to current rather than re-syncing on every run.

A legacy "1" keeps working: require_activation() tests non-emptiness, and the
prologue's [ -e ] fails on it, so it activates once and upgrades.

activate.ps1

@@ -52,6 +52,8 @@ try {
     if ($null -eq $priorPath) { Remove-Item env:PYTHONPATH -ErrorAction SilentlyContinue } else { $env:PYTHONPATH = $priorPath }
     if ($null -eq $priorHome) { Remove-Item env:PYTHONHOME -ErrorAction SilentlyContinue } else { $env:PYTHONHOME = $priorHome }
 }
+# __HERMES_ACTIVATED (the sentinel repo scripts and the shebang prologue read)
+# is part of the composed env, so it is saved and restored with the rest.
 $global:_hermesKeys = @($composed.PSObject.Properties.Name)
 $global:_hermesSaved = @{}
 foreach ($key in $global:_hermesKeys) {