v0.8.2: cleanup + prose + structure + decomposition + budget + errors

Phase 1 — dedup + hardening (~9 items):
- Remove duplicate *skill-registry* defvar from core-skills
- Merge *backend-registry* into *probabilistic-backends*, delete backend-register
- Remove inject-stimulus alias, standardize on stimulus-inject
- Add pre-eval sandbox (skill-source-scan) blocks restricted symbols before eval
- Remove dead plist-get function; remove duplicate json-alist-to-plist export
- Fix read-framed-message whitespace DoS (4096-iteration max)
- Add *read-eval* nil to dispatcher-approvals-process read-from-string (RCE)
- Add test-op to ASDF; update .asd version 0.4.3→0.7.2

Phase 2 — prose + contracts + reorder:
- Split ROADMAP: 2623→1089 lines (TODO only), CHANGELOG: 260→1528 lines (full DONE history, 14 versions reverse chron)
- Add Contracts + Overview to 6 channel files + embedding-native + programming-standards + symbolic-scope
- Reorder 28 .org files: Contract → Test Suite → Implementation (TDD order)
- Add 7-phase inline prose to think() in core-reason
- Expand USER_MANUAL: 183→461 lines (10 new sections)

Phase 3 — decomposition + export organization:
- Decompose think() into think-assemble-prompt, think-call-llm, think-parse-response orchestrator
- Organize 188 exports into 16 grouped sections by module

Phase 4 — budget enforcement + error protocol:
- Per-session budget enforcement (SESSION_BUDGET_USD env var, budget-exhausted-p, guard in think-call-llm)
- Error condition hierarchy (6 conditions: pipeline-error, llm-error, gate-error, budget-error, protocol-error)
- Restarts in loop-process: skip-signal, use-fallback, abort-pipeline

org/channel-slack.org

@@ -7,6 +7,31 @@
 
 Extracted from gateway-messaging in v0.5.0. Isolated platform — Slack-specific poll and send logic.
 
+* Overview
+
+The Slack channel provides bidirectional communication via the Slack Web API
+(chat.postMessage for outbound, conversations.history for inbound polling).
+Messages from Slack channels are injected into the cognitive pipeline as
+~:user-input~ signals with ~:source :slack~. Outbound messages route through
+the actuator registry when the pipeline targets ~:slack~.
+
+The channel uses two functions: ~slack-poll~ (inbound sensor) and ~slack-send~
+(outbound actuator). Both retrieve the bot token from the credentials vault.
+HITL commands are intercepted before injection so approval flows work identically
+across all channels.
+
+** Contract
+
+1. (slack-get-token): returns the Slack bot token from the vault
+   (via ~vault-get-secret :slack~), or nil if not configured.
+2. (slack-poll): polls configured channels via conversations.history,
+   injects each non-bot message as a ~:user-input~ stimulus with
+   ~:source :slack~. Handles API errors gracefully. HITL commands are
+   intercepted before injection.
+3. (slack-send action context): sends a message via chat.postMessage.
+   Extracts ~:channel-id~ and ~:text~ from the action plist. Uses Bearer
+   token authentication. Logs send failures without crashing the pipeline.
+
 * Implementation
 
 #+begin_src lisp