feat: Implement Slack export importer and enhance README documentation for historical context recovery

scripts/slack/README.md

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+# Slack History Import
+
+This directory contains helpers to ingest historical Slack exports as context sources for the workspace.
+
+## Goal
+
+Use old Slack history to recover durable project context, stakeholder relationships, Jira references, and past decisions without treating the raw archive as current truth.
+
+## Recommended Use
+
+- import selected channels, not the whole export blindly
+- prefer recent or relevant historical windows
+- promote only durable, project-relevant facts into workspace memory
+- keep raw imported artifacts under `generated/`
+- keep the raw Slack export under `archives/slack/export/` when you want the workspace to find it later
+- for a first pass on a large export, let the importer auto-detect `fidelity*` channels and auto-tune message selection
+
+## Supported Export Shape
+
+The importer expects the standard Slack export structure:
+
+- one folder per channel
+- one JSON file per day inside the channel folder
+- optional `users.json` at the export root
+
+## Usage
+
+Manual example:
+
+```bash
+python3 scripts/slack/import_slack_export.py \
+  --export-path /absolute/path/to/slack-export \
+  --channel-prefix fidelity \
+  --output-dir scripts/slack/generated
+```
+
+This generates:
+
+- `scripts/slack/generated/slack_context.jsonl`
+- `scripts/slack/generated/slack_summary.md`
+
+Use the OpenCode command `/slack-import ...` for the guided workflow.
+
+## Large Export Behavior
+
+For very large multi-year exports, the importer is designed to be selective by default:
+
+- auto-detects channels whose names start with `fidelity` when no channels are specified
+- auto-tunes the message limit based on archive size
+- prefers recent messages plus older high-signal messages
+- prioritizes Jira IDs, approvals, scope changes, root-cause notes, points, and durable technical patterns
+
+Override behavior if needed:
+
+- `--channels fidelity-preguntas,fidelity-ios`
+- `--all-channels`
+- `--max-messages 8000`
+- `--since 2025-01-01`
+
+## Recommended Archive Location
+
+If you want the workspace to find the export later without needing a custom path each time, place the extracted export here:
+
+```text
+archives/slack/export/
+```
+
+The `/slack-import` command will use that location automatically when no explicit path or environment variable is provided.