Update from code changes: rules-as-substrate doctrine and new rule actions

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Summary

Update the rules and series docs to reflect the rules-as-universal-substrate refactor — new actions, renamed condition fields, and the removal of series detection plus series tags.

Changes

• transactions/rules.mdx — rewritten around the "provider data is immutable; intelligence accrues as rules" doctrine. Adds the full action catalog (assign_series, assign_counterparty, set_metadata, remove_metadata, flag, unflag alongside the existing actions), the renamed condition fields (provider_name, provider_merchant_name, provider_category_primary, provider_category_detailed), the new series / in_series / counterparty / has_counterparty / date-part / metadata.<key> fields, the approx / between numeric operators, the match-stability classification, and last-writer-wins semantics.
• guides/understanding-rules.mdx — adds the doctrine intro, updates every example to the new field names, replaces the threshold-tag example with a flag example, and adds a recurrence-idiom subscription example (amount approx + day_of_month approx + assign_series).
• guides/tracking-subscriptions.mdx — series are now thin, rule-maintained entities (name + type). Removes the detector / candidate-verdict / cadence / expected-amount / next-expected-date model, the add_series_tag / remove_series_tag / review_series / rekey_series / split_series / set_series_type / explain_series_candidates flows, and the inherited-series-tag surface. Replaces the agent surface with the trimmed list_series / get_series / assign_series / update_series / unlink_series_transactions set and points users at adding add_tag to the same rule that does assign_series.
• mcp/tools.mdx — replaces the stale series tools table with the rule-maintained equivalents and adds a new Counterparties subsection mirroring the same model.
• mcp/reference/rules-write.mdx and mcp/reference/rules.mdx — adds the new action shapes, fixes the renamed field names in examples, and rewrites the condition grammar short-form with the match-stability classes, new operators, metadata conditions, and the recurrence idiom.
• guides/breadbox-in-a-nutshell.mdx and guides/single-routine-reviewer.mdx — replace stray references to the old name / merchant_name field names.

Context

Tracks the rules-as-universal-substrate sprint (#1906) and the destructive series-tags removal (#1918) in the source repo.

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