akm tasks: support scheduling tasks from any registered stash, not just the primary stashDir

[itlackey]

Summary

Scheduled tasks (akm tasks …) are bound exclusively to the primary stash
(config.stashDir). Registered source stashes (config.sources[]) are used
for search/curate, but their tasks/ directories are never scanned, and there is
no way to add/sync/run a task from a source stash. This makes it impossible to
version-control task definitions without either a fragile symlink or committing
secrets to git.

Current behavior (akm 0.8.14)

akm config exposes a primary stash plus additional read sources:

{
  "stashDir": "/stash",
  "sources": [
    { "path": "/host-stash",       "name": "host-akm", "writable": true },
    { "path": "/work/agent-stash", "name": "agent-stash" }
  ]
}

• akm tasks sync reconciles only ${stashDir}/tasks with the OS scheduler — it
  accepts no --stash / --source / --all flag.
• akm tasks add writes only to the primary stash — no stash/source target option.
• akm tasks list lists "tasks in the stash" (the primary one).
• There is no AKM_TASKS_DIR (or equivalent) override; tasks always resolve to
  ${AKM_STASH_DIR}/tasks.

A task defined in a source stash's tasks/ directory is therefore invisible to akm.

Why this matters

A common setup separates concerns between two stashes:

• Primary stash (/stash) holds runtime state that must not be in git:
  secrets/, env/ (e.g. env:user credentials), and the akm database/index
  (.akm/).
• A git-tracked source stash (e.g. agent-stash at /work/agent-stash) holds
  the versioned skills / scripts / content.

Scheduled tasks are automation source that logically belongs in the versioned
repo — but because akm binds tasks to stashDir, they can't be versioned there.
The only options today are both bad:

1. Symlink ${stashDir}/tasks → <repo>/tasks. Works, but fragile: a container
   restart / re-seed can replace the symlink with a real directory and silently
   diverge, or the sync loop can transiently see no tasks/ dir.
2. Make the repo the stashDir. This drags secrets/, env/, and the akm DB
   into a git working tree (secret-leak risk) and couples the entire runtime stash
   to the repo.

Neither satisfies the goal: keep secrets out of git, but version the task
definitions.

Recommended changes

1. Scan tasks across all registered stashes (primary + sources) in
   tasks list / sync / run / history, not just stashDir.
2. Per-source opt-in for task execution (security-critical). A source's tasks
   should only be scheduled if the source is explicitly trusted for execution —
   e.g. a tasks: true flag on the source in config (or gate on the existing
   writable: true). Do not auto-run tasks from every registered source by
   default: a source stash defining a cron task is arbitrary code execution on
   every host that registers it.
3. Namespace tasks by stash to avoid id collisions and make provenance clear —
   e.g. agent-stash:reddit-leads. Record the origin stash in the generated cron
   block (# akm:task <stash>/<id> …) and route enable / disable / sync /
   history to the correct stash's file.
4. akm tasks add --stash <name> to create a task in a chosen registered stash
   (defaulting to the primary for back-compat).
5. (Simpler alternative / complement) an AKM_TASKS_DIR env var or
   config.tasksDir override to point the task directory somewhere other than
   ${stashDir}/tasks. This lets an operator relocate tasks to a git-tracked path
   without moving the rest of the stash.

Item (2) is the essential guardrail — opt-in per source keeps this from
becoming a mechanism for an untrusted/shared source stash to run code on every
host that registers it.

Environment

• akm 0.8.14 (also observed as akm-cli@0.9.0-beta.58)
• OS cron scheduler backend, Linux container
• Config: one primary stashDir + filesystem source stashes as shown above