feat(tooling): mise run import-todoist — seed a heph store from Todoist

Turn the one-off Todoist importer into a documented, repeatable mise task.
Self-contained (spawns its own hephd), dry-run by default into a throwaway
store, `-- --commit` writes into the real store after backing it up. Auth via
TODOIST_TOKEN or TODOIST_OP_REF (op://). Mapping per design §6.2.1:
project hierarchy (+ Inbox→unfiled), priority→attention by meaning, due→do-date,
NL recurrence, descriptions + sub-tasks→canonical-context doc.

- mise-tasks/import-todoist
- docs/how-to/import-todoist.md (+ how-to index, reference mise-tasks table)
- changelog fragment

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/reference/reference.md

@@ -57,6 +57,7 @@ Technical reference material for the repository tooling that ships with this pro
 | `mise run docs-check-links` | Validate wiki-links against existing doc filenames |
 | `mise run docs-mikado` | Inspect active Mikado chains and resume C2 work |
 | `mise run docs-preview <tarball>` | Extract and serve a released docs tarball locally |
+| `mise run import-todoist` | Seed a heph store from Todoist (dry-run by default; `-- --commit` to write) — see [[import-todoist]] |
 | `mise run mikado-branch-invariant-check` | Validate `mikado/*` branch commit discipline |
 | `mise run pr-comments <pr_number>` | List unresolved PR comments |
 | `mise run runner-logs [run_number]` | List Forgejo Actions runs or fetch logs for a job |