Add ctail, refactor README

docs/ctail.md

@@ -0,0 +1,57 @@
+# ctail
+
+Tail a Static CT log, printing a one-liner per new certificate or precertificate as it arrives.
+
+## Install
+
+```bash
+GOPRIVATE=git.ipng.ch go install git.ipng.ch/certificate-transparency/ctfetch/cmd/ctail@latest
+```
+
+## Usage
+
+```bash
+ctail [flags] <log-url>
+```
+
+Example:
+
+```bash
+ctail https://halloumi2026h1.mon.ct.ipng.ch
+```
+
+By default `ctail` starts at the current tree tip and prints new entries as they appear. Use `--from-leaf 0` to replay from the beginning.
+
+## Output format
+
+One line per entry:
+
+```
+ leaf-index  type  validity-range            issuer (up to 40 chars)          subject name
+```
+
+Example:
+
+```
+1440154358 cert  2026-03-31..2026-06-29  Let's Encrypt R13                bereavementcounselling.uk
+1440154359 pre   2026-03-31..2026-06-29  ZeroSSL ECC Domain Secured Ce... alpenglowforeverfilms.com
+```
+
+- **type**: `cert` for a final certificate, `pre` for a precertificate
+- **issuer**: CommonName, prefixed with the organisation name when the CN alone is terse (e.g. `R13` → `Let's Encrypt R13`)
+- **subject name**: first DNS SAN, falling back to the certificate's CommonName
+
+## Flags
+
+| Flag | Default | Description |
+|---|---|---|
+| `--interval` | `15s` | How often to poll the checkpoint (minimum 1s) |
+| `--from-leaf` | `-1` | Start from this leaf index; `-1` means current tree tip |
+| `--rate-limit` | `2s` | Minimum time between HTTP requests (minimum 100ms) |
+| `--user-agent` | `ctail/VERSION (https://git.ipng.ch/certificate-transparency/)` | User-Agent header sent with every request |
+
+## Notes
+
+- The interval timer starts when the checkpoint is fetched, so tile-fetch time counts against the interval and the next poll stays on schedule.
+- A tile is only fetched once the checkpoint confirms it is complete (256 entries). This avoids unnecessary 404s at the tree tip.
+- Status and error messages go to stderr; the entry one-liners go to stdout.