feat(list): add --json flag with connect/disconnect timestamps

[noperator]

Adds --json flag to shpool list to expose session data including when sessions were last connected/disconnected, making it easier to identify recently-used sessions.

Closes #292

Supersedes #293, which added optional columns to the tabular output. Per maintainer feedback, pivoted to a single --json flag that outputs all session data as JSON, allowing users to filter/format with jq.

New --json CLI flag:

$ shpool  list --help
lists all the running shell sessions

Usage: shpool list [OPTIONS]

Options:
  -j, --json  Output as JSON
  -h, --help  Print help

Behavior:

• Default output unchanged. Without the flag, shpool list behaves exactly as before.
• JSON output includes all fields: name, started_at_unix_ms, connected_at_unix_ms, disconnected_at_unix_ms, status
• Timestamps are null until the relevant event occurs.

Testing:

# start daemon
./target/debug/shpool -s /tmp/shpool-dev.sock daemon

# open this session, then detach
./target/debug/shpool -s /tmp/shpool-dev.sock attach test1

# default list output unchanged
./target/debug/shpool -s /tmp/shpool-dev.sock list
NAME	STARTED_AT	STATUS
test1	2026-01-23T20:16:12.467+00:00	disconnected

# JSON contains extra session data
./target/debug/shpool -s /tmp/shpool-dev.sock list --json
{
  "sessions": [
    {
      "name": "test1",
      "started_at_unix_ms": 1769199372467,
      "connected_at_unix_ms": 1769199372467,
      "disconnected_at_unix_ms": 1769199373669,
      "status": "Disconnected"
    }
  ]
}

# open another session so we can compare and sort by time
./target/debug/shpool -s /tmp/shpool-dev.sock attach test2

# use jq for custom/complex session sorting (in this case, by recent activity)
./target/debug/shpool -s /tmp/shpool-dev.sock list --json |
    jq '.sessions | sort_by([
        ({ "Disconnected": 0, "Attached": 1 }[.status]),
        (if .status == "Attached" then .connected_at_unix_ms else .disconnected_at_unix_ms end)
    ])'
[
  {
    "name": "test1",
    "started_at_unix_ms": 1769199372467,
    "connected_at_unix_ms": 1769199372467,
    "disconnected_at_unix_ms": 1769199373669,
    "status": "Disconnected"
  },
  {
    "name": "test2",
    "started_at_unix_ms": 1769199432284,
    "connected_at_unix_ms": 1769199432284,
    "disconnected_at_unix_ms": null,
    "status": "Attached"
  }
]

[ethanpailes]

This is great! just a few bits of feedback and then we should be ready to merge.

I'll also create a bug as a reminder to drop the STARTED_AT column from the normal list output before 1.0. I rather doubt people care about that most of the time, and now that there is a better way to deal with the data we should probably clean that up the next time we do a compatibility break.

[noperator]

Made requested changes:

• Rename connected_at -> last_connected_at, disconnected_at -> last_disconnected_at
• Combine timestamps into SessionLifecycleTimestamps behind single Mutex
• Update help text to "Output as JSON, includes extra fields"

Repeated tests with new changes

# start daemon
./target/debug/shpool -s /tmp/shpool-dev.sock daemon

# open this session, then detach
./target/debug/shpool -s /tmp/shpool-dev.sock attach test1

# default list output unchanged
./target/debug/shpool -s /tmp/shpool-dev.sock list
NAME	STARTED_AT	STATUS
test1	2026-01-23T21:59:51.955+00:00	disconnected

# JSON contains extra session data
./target/debug/shpool -s /tmp/shpool-dev.sock list --json
{
  "sessions": [
    {
      "name": "test1",
      "started_at_unix_ms": 1769205591955,
      "last_connected_at_unix_ms": 1769205591955,
      "last_disconnected_at_unix_ms": 1769205597359,
      "status": "Disconnected"
    }
  ]
}

# open another session so we can compare and sort by time
./target/debug/shpool -s /tmp/shpool-dev.sock attach test2

# use jq for custom/complex session sorting (in this case, by recent activity)
./target/debug/shpool -s /tmp/shpool-dev.sock list --json |
    jq '.sessions | sort_by([
        ({ "Disconnected": 0, "Attached": 1 }[.status]),
        (if .status == "Attached" then .last_connected_at_unix_ms else .last_disconnected_at_unix_ms end)
    ])'
[
  {
    "name": "test1",
    "started_at_unix_ms": 1769205591955,
    "last_connected_at_unix_ms": 1769205591955,
    "last_disconnected_at_unix_ms": 1769205597359,
    "status": "Disconnected"
  },
  {
    "name": "test2",
    "started_at_unix_ms": 1769205633087,
    "last_connected_at_unix_ms": 1769205633087,
    "last_disconnected_at_unix_ms": null,
    "status": "Attached"
  }
]

[ethanpailes]

Sorry to ask for more changes on a second round, but I just realized the change itself has no new tests. Do you mind adding a new test to sanity check in shpool/tests/list.rs. It doesn't have to be fancy, just lauching a sub-process and grepping the output for one of the new fields and maybe using serde_json to verify that the output is valid json is enough.