feat(pi): add session resume shorthand (#113)

docs/developing.md

@@ -45,7 +45,7 @@ The core handler module containing:
 
 **Key patterns:**
 - Progress edits are best-effort and only run when new events arrive (Telegram outbox handles rate limiting/coalescing)
-- Resume tokens are runner-formatted command lines (e.g., `` `codex resume <token>` ``, `` `claude --resume <token>` ``, `` `pi --session <path>` ``)
+- Resume tokens are runner-formatted command lines (e.g., `` `codex resume <token>` ``, `` `claude --resume <token>` ``, `` `pi --session <id>` ``)
 - Resume lines are stripped from the prompt before invoking the runner
 - Errors/cancellation render final status while preserving resume tokens when known
 
@@ -356,7 +356,7 @@ transport.send()/edit() final message, delete progress if needed
 ### Resume Flow
 
 Same as above; auto-router polls all runners to extract resume tokens:
-- Router returns first matching token (e.g. `` `claude --resume <id>` `` routes to Claude, `` `pi --session <path>` `` routes to Pi)
+- Router returns first matching token (e.g. `` `claude --resume <id>` `` routes to Claude, `` `pi --session <id>` `` routes to Pi)
 - Selected runner spawns with resume (e.g. `codex exec --json resume <token> -`, `pi --print --mode json --session <path> <prompt>`)
 - Per-token lock serializes concurrent resumes on the same thread
 

docs/runner/pi/pi-runner.md

@@ -31,13 +31,15 @@ Provide the **`pi`** engine backend so Takopi can:
 Takopi appends a **single backticked** resume line at the end of the message, like:
 
 ```text
-`pi --session /home/user/.pi/agent/sessions/--repo--/2026-01-02T12-34-56-789Z_abcd.jsonl`
+`pi --session ccd569e0`
 ```
 
 Notes:
 
 * `pi --resume/-r` opens an interactive session picker, so Takopi uses `--session <path>` instead.
-* The resume token is the **session file path** (JSONL), treated as an opaque string.
+* The resume token is the **session id** (short prefix), derived from the first JSON
+  object in the session file. If the id cannot be read, Takopi falls back to the
+  session file path.
 * If the path contains spaces, the runner will quote it.
 
 ### Non-interactive runs

docs/runner/pi/pi-takopi-events.md

@@ -30,10 +30,11 @@ Notes:
 - Canonical resume line (embedded in chat):
 
 ```
-`pi --session <path>`
+`pi --session <id>`
 ```
 
-The token is the **session JSONL file path**.
+The token is the **short session id**, derived from the first JSON object in the
+session file. If the id cannot be read, Takopi falls back to the session file path.
 
 Why not `--resume`?
 - `--resume/-r` opens an interactive session picker; it does not accept a

docs/specification.md

@@ -41,7 +41,7 @@ The canonical ResumeLine embedded in chat MUST be the engine’s CLI resume comm
 
 - `codex resume <id>`
 - `claude --resume <id>`
-- `pi --session <path>`
+- `pi --session <token>`
 
 ResumeLine MUST resume the interactive session when the engine offers both interactive and headless modes. It MUST NOT point to a headless/batch command that requires a new prompt (e.g., a `run` subcommand that errors without a message).