Plan usage proof cmds

[Pranav-1100]

This PR brings the management commands from the JS client over to Guppy so we can check subscriptions, usage, and local proofs straight from the terminal. Resolves #333.

Here is a quick breakdown of what was added:

• guppy plan get and guppy plan set:
  Uses plan/get capability to show the current billing plan. If no billing profile exists, it catches the error and asks user to visit the console to set one up. I also added (plan set ) so users can change plans directly from the terminal.

• guppy usage report:
  Uses the usage/report capability to get the total bytes used by a space. It fetches data starting from 2020 (just like the JS client) to make sure it counts all historical uploads.

• guppy proof ls:
  A local command that looks into the agent's datastore and lists all the saved UCAN delegations, showing the CID, Issuer, and exact capabilities (like * or ucan/attest).

Verification with Output

1. Plan Get / Set

$ guppy plan get
Plan Details for did:mailto:gmail.com:prana...:
  Product: did:web:starter.storacha.network
  Limit:   0
  Updated: 2026-02-18T13:15:32.955Z

$ guppy plan set did:web:lite.storacha.network   
Setting plan for did:mailto:gmail.com:prana... to did:web:lite.storacha.network...
Success! Plan updated.

$ guppy plan get                               
Plan Details for did:mailto:gmail.com:prana...:
  Product: did:web:lite.storacha.network
  Limit:   0
  Updated: 2026-02-21T05:49:59.693Z

2. Usage Report

$ guppy usage report did:key:z6Mke...
Fetching usage for space did:key:z6Mke...
---------------------------------------------------
Provider:    did:web:up.storacha.network
Total Usage: 251 MiB
---------------------------------------------------

3. Proof LS

$ guppy proof ls
Found 7 proof(s):
CID:    bafyreiej...
Issuer: did:web:up.forge.storacha.network
Capabilities:
  - ucan/attest (with: did:web:up.forge.storacha.network)
CID:    bafyreifn...
Issuer: did:key:z6Mku...
Capabilities:
  - * (with: did:key:z6Mku...)
...